What is Isomorphic JavaScript and When Should it Be Used?

Lately I have been reading more and more recommendations for isomorphic JavaScript in single page applications (SPAs). My last Blog post was a response to some folks in my galaxy of influence supporting server-side rendering tactics. The combination of client and server rendering is known as Isomorphic JavaScript. Isomorphic JavaScript is one of those fancy sounding computer science terms. Isomorphic means the application uses the same rendering engine on the client and the server, thus making it easier for developers to maintain markup templates.