This term (and paradigm) comes from the historian of science Thomas Kuhn. A paradigm is the set of unquestioned ideas scientists share when doing normal science. A paradigm shift is when a paradigm fails radically to explain the data, and instead of changing it a bit, science has to make up something new. This is also known as revolutionary science, or a scientific revolution.

Sample paradigms and paradigm shifts.

This term is abused and used out of context all the time.