xUnit is a label used for an automated testing software framework that shares significant structure and functionality that is traceable to a common progenitor SUnit.
The SUnit framework was ported to Java by Kent Beck and Erich Gamma as JUnit which gained wide popularity. Adaptations to other languages were also popular which led some to claim that the structured, object-oriented style works well with popular languages including Java and C#.
The name of an adaptation is often a variation of "SUnit" with the "S" replaced with an abbreviation of the target language name. For example, JUnit for Java and RUnit for R. The term "xUnit" refers to any such adaptation where "x" is a placeholder for the language-specific prefix.
The xUnit frameworks are often used for unit testing – testing an isolated unit of code – but can be used for any level of software testing including integration and system.