It's not just you. It's too abstract of a word to tell me anything without more context.

Edit: Here's the definition they're using: http://nerds.airbnb.com/isomorphic-javascript-future-web-app.... In short, "JavaScript that runs on both the client and the server." This goes beyond "using node" to mean that (e.g.) everything is rendered on the server, but that you can also render only parts and send them to the client to handle on its own.