Yes, an isomorphic js program is a program that can run both in browsers and nodejs. I know that is not mathematically correct, but lot of devs use this word and understand its meaning.
The term 'isomorphic' refers to the output, as in being able to render the same UI / application on the server and on the client.
It’d be easy to be active and not come across it though. Most of the JavaScript world maintains a clear client/server separation. In my experience most production apps can’t share code between client and server. It’s either written and packaged for one or the other.