Strictly speaking, a web application is a special case of client-server application. However, people shy away from that term because of some of the historical baggage. (In the past, a client-server application would need a separate step to deploy the client, and it would be up to the developer to think about the concurrency of connections to the server. These area of concerns don’t apply when we consider the browser as being a sort of super-client that handles all this infrastructure.) Today, some people use client-server to describe web applications, and some draw a distinction with web applications.

An isomorphic app is basically an app that you design in one “piece,” but can then be executed on the server or client as needed. The idea is that the page might start out being rendered by the web server and then be refreshed by the same code running on the web browser. The overall goal is to simplify your programming model (really, the execution model and division of work between browser and server stays the same as it would be for a modern single-page web app that isn’t isomorphic).