Sorry … had to jet off to a meeting.
My definition of a Citizen Developer takes two forms: Formal and Informal. But, they start with the main emphasis that they are an end user. They don’t work for IT or a Dev Team. They have a job to do and will seek out ways to increase their efficiency and efficacy.
Informal: A user that needs to get things done and uses the tools at hand to get it done. Much more rogue in that they need to get their job done and they have found / created shortcuts.
They will share their creation with the team, but may not be willing to make it pretty (UX), maintain it, update, manage, etc. It’s not that they are bad people they just have a job to do and will do what it takes to get it done. (Of course, this can lead to challenges. But, that’s not their problem — management can implement new goals and roles if needed)
Formal: A user that is more process oriented and willing to share their learnings / creations with others. They will use these skills in combination with the team, IT (if needed), and others to make the system(s) run more efficiently.
Perhaps these are what Mark and Marc call Power Users. But, to me a Power User might be a Citizen Developer with a more narrow focus. They really know SharePoint or PowerBI or T-SQL or some other product or service inside and out.