It means that it can be used on client and server. Its a term coined by (I believe) the nodejitsu people in this article [0]. Airbnb also used the term to describe their rendr library [1] which aims to allow exactly that: rendering on both client and server

[0] http://blog.nodejitsu.com/scaling-isomorphic-javascript-code...

[1] http://nerds.airbnb.com/isomorphic-javascript-future-web-app...