How to define a web app?

What essentially defines a web app? Are all useful websites web apps?

As examples, is Google search a web app? G-mail? Wikipedia?

From web app Vaibhav Garg

• I think that in one phrase it can be defined as an application that you run and access through your browser. I guess that the difference with a "basic" website would be that the website aim is mostly to display information and not really interact with the user.
  Of course the definition is not really precise. For some application it is quite obvious (as for most of Google products, facebook...), but for others it may depend on your own interpretation.

  I'd cite some definitions found on websites :

  • this site's FAQ:

  Web Apps - Stack Exchange is for expert and advanced users of web applications, so if your question generally covers …

  * Gmail
  * Facebook
  * Twitter
  * Google
  

  … or any other website which behaves like an application, then you're in the right place to ask your question!

  • Wikipedia is a little more generic :

  A web app is an application that uses technologies such as Javascript, CSS and HTML5 and is executed in a web browser. The application can be run directly from a website, or, if it uses the HTML 5 Offline Application Cache and Web Storage, can be downloaded and installed locally, for offline use.

  or

  In system software, a web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application that is hosted in a browser-controlled environment (e.g. a Java applet) or coded in a browser-supported language (such as JavaScript, combined with a browser-rendered markup language like HTML) and reliant on a common web browser to render the application executable.

  From Julien N