A Doctor of Philosophy (PhD, Ph.D., or DPhil; Latin: philosophiae doctor or doctor philosophiae) is the most common degree at the highest academic level awarded following a course of study and research. PhDs are awarded for programs across the whole breadth of academic fields. Because it is an earned research degree, those studying for a PhD are required to produce original research that expands the boundaries of knowledge, normally in the form of a dissertation, and defend their work before a panel of other experts in the field.

The completion of a PhD is often a requirement for employment as a university professor, researcher, or scientist in many fields.Individuals who have earned a Doctor of Philosophy degree use the title Doctor (often abbreviated "Dr" or "Dr.") with their name, although the proper etiquette associated with this usage may also be subject to the professional ethics of their own scholarly field, culture, or society. Those who teach at universities or work in academic, educational, or research fields are usually addressed by this title "professionally and socially in a salutation or conversation."Alternatively, holders may use post-nominal letters such as "Ph.D.", "PhD", or "DPhil" (depending on the awarding institution). It is, however, considered incorrect to use both the title and post-nominals at the same time.