Doctor of Philosophy, abbreviated PhD or Ph.D. for the Latin philosophiæ doctor, meaning ''teacher of philosophy'', is advanced academic degree awarded by universities. The PhD is in many English-speaking countries the highest degree one can earn and applies to graduates in a wide array of disciplines in the sciences and humanities. The PhD or equivalent has become a requirement for a career as a university professor or researcher in most fields.

The detailed requirements for award of a PhD degree vary throughout the world. In some countries, most universities require coursework in addition to research for PhD degrees. A candidate must submit a thesis or dissertation consisting of a suitable body of original academic research, which is in principle worthy of publication in a peer-refereed context.

In many countries a candidate must defend this work before a panel of expert examiners appointed by the university; in other countries, the dissertation is examined by a panel of expert examiners who stipulate whether the dissertation is in principle passable and the issues that need to be addressed before the dissertation can be passed.