> What is it isomorphic to?

Don't bother, most people using this word in the context of javascript apps do not understand what it really means. It just has became a buzz word meaning :" you can render your views on client and on the server using JavaScript". IE you can render an HTML page representing the initial state of your app, then upgrade to an interactive application using JavaScript on the front-end seamlessly .

You can't do that with angularjs 1.x for instance without some kind of heavy machinery relying on a headless browser. That's why angular 2.x basically ditches 1.x and is a completely different framework.

To be fair, it comes from the same people who call node.js "close to the metal"

In javascript frameworks, isomorphic usually refers to running the same code in the server (node.js) as well as the client.