XUnit is the family name given to bunch of testing frameworks that have become widely known amongst software developers. The name is a derivation of JUnit, the first of these to be widely known.
The origins of these frameworks actually started in Smalltalk. Kent Beck was a big fan of automated testing at the heart of software development. To help him, and his clients, do this he would build a simple framework to organize and run unit tests. The focus was on making it easy for programmers to define the tests using their regular smalltalk environment, and then to run either a subset or a full set of tests quickly. Kent and his followers would run unit tests after every change to the system going through a rapid edit and test cycle in the Smalltalk IDE.
I ran into Kent at this time. I'd already done the same thing myself, but Kent's framework had a nice combination of absurd simplicity and just the right features for me. Basically he did a better job of it than I did so I just used his. In particular we used the framework on C3, where Ron Jeffries was also introduced to it.