Isomorphic is accurate. Progressive is just plain wrong
Isomorphic might sound weird at first, but it is more accurate in literal meaning. Progressive sounds familiar in web development but is just plain wrong.
On the other hand, the term “progressive enhancement” refers to techniques that attempt to utilise the potential of modern browsers while affecting the user experience as less as possible in the not-so-modern browsers. That’s why it is called progressive enhancement. It is an enhancement, that one would like to have but one could live without.
(For instance, imagine you have a link. The user clicks the link and it opens in a browser tab. You may hijack the click event on the link and make it open in a custom popup or iframe or whatever. You progressively enhanced it, but it will still work if you hadn’t.)