In the JS community, it means a library that can run client-side (in the browser) or server-side (in NodeJS).

Right. While I understand why they use the term this way, it still feels semantically abusive given the math concept.

"Isomorphism" means different things in different communities https://en.wikipedia.org/wiki/Isomorphism_(disambiguation)

Why is the definition in one community special?

Isomorphism means "same shape" from Greek, sounds like a good way to describe code written in the same language running in different environments.

Not that you're wrong, but it's a deep and pervasive concept in mathematics, and the field of computing is closely connected to mathematics in general. It's sort of a namespace collision.

Ok, but the same arguments could be made about the terms "refactor" and "plane" (as in control-plane/data-plane) but we seem to manage to cope with the overload.

On one hand, you're right. On the other hand, English is basically never consistent, so why should this be an exception?

This isn't an argument about consistency. It's an argument about whether overloaded terms are acceptable. If you want to argue that terms shouldn't be consistent that's an entirely different (and entirely bizarre, IMO) discussion.

Sorry, that was just meant as a dumb joke.

Another term in computing/programming for "same code in different environments" is "portable". (e.g. C++ code that compiles/runs on both Linux/Windows). I've not seen 'isomorphic' used in computing outside of JS.

I suspect you might have a personal bias, though.

The community has been slowly moving to the term "Universal JS" for that concept, though. Probably partly due to the mathematical ambiguity, but mostly because "universal" is a bit more approachable term.

So, it means cross platform.

Specifically in this context it means that the same JS code can be run in the server and in the browser.