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Andreas Antonopolous, author of the book “Mastering Ethereum”
Taken from the book Mastering Ethereum

An object containing an address, balance, nonce and optional storage and code.
An account can be a contract account or an EOA (externally owned account).

Most generally, this represents an EOA or contract that can receive (destination
address) or send (source address) transactions on the blockchain. More specifically, it
is the right-most 160 bits of a Keccak hash of an ECDSA public key.

In Solidity, assert(false) compiles to 0xfe, an invalid opcode, which uses up all
remaining gas and reverts all changes. When an assert() statement fails, something
very wrong and unexpected should be happening, and you will need to fix your code.
You should use assert to avoid conditions which should never, ever occur.

A positional number representation where the most significant digit is first. The opposite
of little-endian, where the least significant digit is first.

Bitcoin Improvement Proposals. A set of proposals that members of the Bitcoin
community have submitted to improve Bitcoin. For example, BIP-21 is a proposal to
improve the Bitcoin uniform resource identifier (URI) scheme.

A block is a collection of required information (a block header) about the comprised
transactions, and a set of other block headers known as ommers. It is added to the
Ethereum network by miners.

In Ethereum, a sequence of blocks validated by the proof-of-work system, each linking
to its predecessor all the way to the genesis block. This varies from the Bitcoin protocol
in that it does not have a block size limit; it instead uses varying gas limits.

Abstract instruction set designed for efficient execution by a software interpreter or a
virtual machine. Unlike human readable source code, bytecode is expressed in numeric

Byzantium fork
Byzantium is the first of two hard forks for the Metropolis development stage. It included
EIP-649: Metropolis Difficulty Bomb Delay and Block Reward Reduction, where the Ice
Age (see below) was delayed by 1 year, and the block reward was reduced from 5 to 3

Converting code written in a high-level programming language (e.g. Solidity) into a
lower level language (e.g. EVM bytecode).

When numerous nodes, usually most nodes on the network, all have the same blocks in
their locally validated best block chain. Not to be confused with "consensus rules".

Consensus rules
The block validation rules that full nodes follow to stay in consensus with other nodes.
Not to be confused with "consensus".

The second part of the Metropolis stage, planned for mid-2018. Expected to include a
switch to hybrid Proof-of-Work/Proof-of-Stake consensus algorithm, among other

Contract account
An account containing code that executes whenever it receives a transaction from
another account (EOA or contract).

Contract creation transaction
A special transaction, with the "zero address" as the recipient, that is used to register a
contract and record it on the Ethereum blockchain (see "zero address").

Decentralized Autonomous Organization. Companies and other organizations which
operate without hierarchical management. Also may refer to a contract named "The
DAO" launched on 30th April 2016, which was then hacked in June 2016 and ultimately
motivated a hard fork (codenamed DAO) at block #1,192,000 which reversed the
hacked DAO contract, and caused Ethereum and Ethereum Classic to split into two
competing systems.

Decentralized Application. At a minimum, it is a smart contract and a web
user-interface. More broadly, a DApp is a web application that is built on top of open,
decentralized, peer-to-peer infrastructure services. In addition, many DApps include
decentralized storage and/or message protocol and platform.

Non-fungible token (NFT) standard introduced by the ERC721 proposal. Unlike ERC20
tokens, deeds prove ownership and are not interchangeable, though they are not
recognized as legal documents in any jurisdiction, at least not currently (see also

A network-wide setting that controls how much computation is required to produce a
proof of work.

Digital signature
A digital signing algorithm is a process by which a user can produce a short string of
data called a "signature" of a document using a private key such that anyone with the
corresponding public key, the signature, and the document can verify that (1) the
document was "signed" by the owner of that particular private key, and (2) the
document was not changed after it was signed.

Elliptic Curve Digital Signature Algorithm, or ECDSA, is a cryptographic algorithm used
by Ethereum to ensure that funds can only be spent by their owners.

Ethereum Improvement Proposals describe proposed standards for the Ethereum
platform. An EIP is a design document providing information to the Ethereum
community, describing a new feature or its processes or environment. For more
information, see (see also "ERC").

In the context of cryptography, lack of predictability, or level of randomness. When
generating secret information, such as private keys, algorithms usually rely on a source
of high entropy to ensure the output is unpredictable.

Ethereum Name Service. For more information, see .

Externally Owned Account. Accounts created by or for human users of the Ethereum

Ethereum Request for Comments, a label given to some EIPs which attempt to define a
specific standard of Ethereum usage.

A Proof-of-Work algorithm for Ethereum 1.0. For more information, see .

Ether is the native cryptocurrency used by the Ethereum ecosystem, which covers gas
costs when executing Smart Contracts. Its symbol is Ξ, the Greek uppercase Xi

An event allows the use of EVM logging facilities. DApps can listen for events and use
them to trigger JavaScript callbacks in the user interface. For more information, see .

Ethereum Virtual Machine, a stack-based virtual machine which executes bytecode. In
Ethereum, the execution model specifies how the system state is altered given a series
of bytecode instructions and a small tuple of environmental data. This is specified
through a formal model of a virtual state machine.

EVM assembly language
A human-readable form of EVM bytecode.

Fallback function
A default function called in the absence of data or a declared function name.

A service that dispenses funds in the form of free test ether that can be used on a

A denomination of ether. 10 ** 15 finney = 1 ether.

This term assumes two main meanings: a change in protocol causing the creation of an
alternative chain, or a temporal divergence in two potential block paths during mining.

The initial test development stage of Ethereum, which lasted from July 2015 to March

Personal Ethereum blockchain which you can use to run tests, execute commands, and
inspect state while controlling how the chain operates.

A virtual fuel used in Ethereum to execute smart contracts. The Ethereum Virtual
Machine uses an accounting mechanism to measure the consumption of gas and limit
the consumption of computing resources (see "Turing complete").

Gas limit
The maximum amount of gas a transaction or block may consume.

Gavin Wood
Gavin Wood is a British programmer who is the co-founder and former CTO of
Ethereum. In August 2014 he proposed Solidity, a contract-oriented programming
language for writing smart contracts.

Genesis block
The first block in a blockchain, used to initialize a particular network and its

Go Ethereum. One of the most prominent implementations of the Ethereum protocol,
written in Go.

Hard fork
A hard fork, also known as a Hard-Forking Change, is a permanent divergence in the
blockchain; one commonly occurs when non-upgraded nodes can’t validate blocks
created by upgraded nodes that follow newer consensus rules. Not to be confused with
fork, soft fork, software fork or Git fork.

A fixed-length fingerprint of variable-size input, produced by a hash function.

HD wallet
A wallet using the Hierarchical Deterministic (HD Protocol) key creation and transfer
protocol (BIP32).

HD wallet seed
An HD wallet seed, or seed, is a value used to generate the master private key and
master chain code for an HD wallet. The wallet seed can be represented by mnemonic
words, making it easier for humans to copy, backup and restore private keys.

The second development stage of Ethereum, launched in March 2016 at block

Ice Age
A hard fork of Ethereum at block #200,000 to introduce an exponential difficulty
increase (aka Difficulty Bomb), motivating a transition to Proof-of-Stake.

IDE (Integrated Development Environment)
An integrated user interface that typically combines a code editor, compiler, runtime,
and debugger.

Immutable Deployed Code Problem
Once a contract’s (or library’s) code is deployed it becomes immutable. Standard
software development practices rely on being able to fix possible bugs and add new
features, so this represents a challenge for smart contract development.

Inter-exchange Client Address Protocol (ICAP)
An Ethereum Address encoding that is partly compatible with the International Bank
Account Number (IBAN) encoding, offering a versatile, checksummed and interoperable
encoding for Ethereum Addresses. ICAP addresses use a new IBAN pseudo-country
code: XE, standing for "eXtended Ethereum", as used in non-jurisdictional currencies
(e.g. XBT, XRP, XCP).

Internal transaction (also "message")
A transaction sent from a contract account to another contract account or an EOA.

The Inter Planetary File System is a protocol, a network and an open-source project
designed to create a content-addressable, peer-to-peer method of storing and sharing
hypermedia in a distributed file system.

Cryptographic hash function used in Ethereum. Keccak256 was standardized as SHA-3.

Key Derivation Function (KDF)
Also known as a "password stretching algorithm", it is used by keystore formats to
protect against brute-force, dictionary, and rainbow table attacks on passphrase
encryption, by repeatedly hashing the passphrase.

Keystore File
A JSON-encoded file that contains a single (randomly generated) private key, encrypted
by a passphrase for extra security.

LevelDB is an open source on-disk key-value store, implemented as a light-weight,
single-purpose library, with bindings to many platforms.

A library in Ethereum is a special type of contract that has no payable functions, no
fallback function, and no data storage. Therefore, it cannot receive or hold ether, or
store data. A library serves as previously deployed code that other contracts can call for
read-only computation.

Lightweight client
A lightweight client is an Ethereum client that does not store a local copy of the
blockchain, or validate blocks and transactions. It offers the functions of a wallet and
can create and broadcast transactions.

Merkle Patricia Tree
A data structure used in Ethereum to efficiently store key-value pairs.

An internal transaction that is never serialized and only sent within the EVM.

Message Call
The act of passing a message from one Account to another. If the destination account is
associated with EVM Code, then the VM will be started with the state of said Object and
the Message acted upon.

Metropolis Stage
Metropolis is the third development stage of Ethereum, launched in October 2017.

Mastering Ethereum Token. An ERC20 token used for demonstration in this book.

A network node that finds valid proof of work for new blocks, by repeated hashing.

The first Ethereum-enabled browser, built by the Ethereum Foundation. It contains a
browser based wallet that was the first implementation of the ERC20 token standard
(Fabian Vogelsteller, author of ERC20, was also the main developer of Mist). Mist was
also the first wallet to introduce the camelCase checksum (EIP-55, see [eip55] ). Mist
runs a full node, and offers a full DApp browser with support for Swarm-based storage
and ENS addresses.

Referring to the Ethereum network, a peer-to-peer network that propagates transactions
and blocks to every Ethereum node (network participant).

A non-fungible token (also known as a "deed"). This is a token standard introduced by
the ERC721 proposal. NFTs can be tracked and traded, but each token is unique and
distinct; they are not interchangeable like ERC20 tokens. NFTs can represent
ownership of digital or physical assets.

A software client that participates in the network.

In cryptography, a value that can only be used once. There are two types of nonce used
in Ethereum. (1) An account nonce: A transaction counter in each account, which is
used to prevent replay attacks. (2) Proof of work nonce: The random value in a block
that was used to satisfy the proof of work.

A child block of an ancestor that is not itself an ancestor. When a miner finds a valid
block, another miner may have published a competing block which is added to the tip of
the blockchain. Unlike Bitcoin, orphaned blocks in Ethereum can be included by newer
blocks as ommers and receive a partial block reward. The term "ommer" is the preferred
gender-neutral term for the sibling of a parent node, but is also sometimes referred to as
an "uncle".

One of the most prominent interoperable implementations of the Ethereum client

Private key
See "Secret Key".

Proof-of-Stake (PoS)
Proof-of-Stake is a method by which a cryptocurrency blockchain protocol aims to
achieve distributed consensus. Proof-of-Stake asks users to prove ownership of a
certain amount of cryptocurrency (their "stake" in the network) in order to be able to
participate in the validation of transactions.

Proof-of-Work (PoW)
A piece of data (the proof) that requires significant computation to find. In Ethereum,
miners must find a numeric solution to the Ethash algorithm that meets a network-wide
difficulty target.

Public key
A number, derived via a one-way function from a private key, which can be shared
publicly and used by anyone to verify a digital signature made with the corresponding
private key.

Data returned by an Ethereum client to represent the result of a particular transaction,
including a hash of the transaction, its block number, the amount of gas used and, in
case of deployment of a Smart Contract, the address of the Contract.

Re-entrancy attack
An attack that consists of an Attacker contract calling a Victim contract function in such
a way that during execution the Victim calls the Attacker contract again, recursively.
This can result, for example, in the theft of funds by skipping parts of the Victim contract
that update balances or count withdrawal amounts.

An amount of ether included in each new block as a reward by the network to the miner
who found the Proof-of-Work solution.

Recursive Length Prefix (RLP)
An encoding standard designed by the Ethereum developers to encode and serialize
objects (data structures) of arbitrary complexity and length.

Satoshi Nakamoto
The name used by the person or people who designed Bitcoin, created its original
reference implementation, and were the first to solve the double-spend problem for
digital currency. Their real identity remains unknown.

A computer programming term that describes an object of which only a single instance
can exist.

Secret key (aka private key)
The secret number that allows Ethereum users to prove ownership of an account or
contracts, by producing a digital signature (see public key, address, ECDSA).

The Secure Hash Algorithm (SHA) is a family of cryptographic hash functions published
by the National Institute of Standards and Technology (NIST).

The fourth and final development stage of Ethereum. Serenity does not yet have a
planned release date.

A procedural (imperative) smart contract programming language with syntax similar to

Smart contract
A program which executes on the Ethereum computing infrastructure.

A procedural (imperative) programming language with syntax that is similar to
JavaScript, C++ or Java. The most popular and most frequently used language for
Ethereum smart contracts. Created by Gavin Wood (co-author of this book).

Solidity inline assembly
EVM assembly language in a Solidity program. Solidity’s support for inline assembly
makes it easier to write certain operations.

Spurious Dragon
A hard fork of the Ethereum blockchain, which occurred at block #2,675,000 to address
more denial of service attack vectors, and another state clearing; see "Tangerine
Whistle". Also, a replay attack protection mechanism.

A decentralized (P2P) storage network, used along with Web3 and Whisper to build

A denomination of ether. 10 12 szabo = 1 ether.

Tangerine Whistle
A hard fork of the Ethereum blockchain, which occurred at block #2,463,000 to change
the gas calculation for certain I/O-intensive operations and to clear the accumulated
state from a denial of service attack, which exploited the low gas cost of those

Short for "test network", a network used to simulate the behavior of the main Ethereum

Data committed to the Ethereum Blockchain signed by an originating account, targeting
a specific address. The transaction contains metadata such as the gas limit for the

One of the most commonly used Ethereum Development Frameworks.

Turing complete
A system of data-manipulation rules (such as a computer’s instruction set, a
programming language, or a cellular automaton) is said to be "Turing complete" or
"computationally universal" if it can be used to simulate any Turing machine. The
concept is named after English mathematician and computer scientist Alan Turing.

Vitalik Buterin
Vitalik Buterin is a Russian–Canadian programmer and writer primarily known as the
co-founder of Ethereum and as the co-founder of Bitcoin Magazine.

A high-level programming language, similar to Serpent, with Python-like syntax.
Intended to get closer to a pure-functional language. Created by Vitalik Buterin.

Software that holds secret keys. Used to access and control Ethereum accounts and
interact with Smart Contracts. Keys need not be stored in a wallet, and can instead be
retrieved from an offline storage (e.g. …

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In a nutshell

Getting more technical…


Daniel Martins

Blockchain enthusiast. See

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