"Usability testing" can be used in a general or a specific way. The narrow definition of a usability test is that a test should identify non-functional issues which could prevent the successful use of a particular product. In broader terms, and via the GAT platform, you might:
Usability testing is generally considered a more specific term than UX or User testing.
UX testing is a superset of usability testing: a UX test might refer to any test which bears an impact on your UX. Lots of feedback on UX might be useful for a product team to hear, but not a usability issue per se if it does not affect users’ levels of success.
User testing is testing on users. Functionality, content, aesthetics, in addition to usability testing can all be covered by User testing; and user testing can also encompass a variety of methods which include but are not limited to the controlled experiments we’d associate with a usability test.