What is difference between web application server and web application

I'm wondering if web application server and web application are different or the same. (not Web server and Application server.)

I thought that web application server is something like tomcat, and web application is like tons of websites.

However oneday my colleage said "web application server is not a thing connecting backend application to server but it's an application itself." So now I got confused.

Maybe the backend application he mentioned is not the same as web application I guess? If then, what is backend application in this context? and what is that mean Web application server is an application itself?

Pls help me to get these concepts. Thanks :)