Action principles were developed by trial and error over three centuries; the names of the principles are not self-describing. Richard Feynman, through his PhD thesis and later through his reinvention of the undergraduate physics course, reinvigorated the field of variational principles in physics. In the process he upended the terminology. Feynman called Hamilton's principal function simply the "action" and Hamilton's principle he called "the principle of least action". The table below summarizes the key terminology found in modern physics literature.