A priest or priestess is a person having the authority or power to administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities. Their office or position is the priesthood, a term which may also apply to such persons collectively.

Priests and priestesses have been known since the earliest of times and in the simplest societies. They exist in all or some branches of Judaism, Christianity, Shintoism, Hinduism, and many other religions, as well, and are generally regarded as having good contact with the deity or deities of the religion to which they subscribe, often interpreting the meaning of events, performing the rituals of the religion, and to whom other believers often will turn for advice on spiritual matters.

In many religions, being a priest or priestess is a full-time job, ruling out any other career. In other cases it is a part-time role. For example in the early history of Iceland the chieftains were entitled goði, a word meaning "priest". But as seen in the saga of Hrafnkell Freysgoði, being a priest consisted merely of offering periodic sacrifices to the Norse gods and goddesses; it was not a full time job, nor did it involve ordination.

In some religions, being a priest or priestess is by human election and/or human choice. In others the priesthood is inherited in familial lines.

Terminology
The English word priest is ultimately from Greek via Latin presbyter, the term for "elder", especially elders of Jewish or Christian communities in Late Antiquity. It is possible that the Latin word was loaned into Old English and only from Old English reached other Germanic languages, giving Old Icelandic prestr, Old Swedish präster, Old High German priast, via the Anglo-Saxon mission to the continent. However, Old High German also has the disyllabic priester, priestar, apparently derivied from Latin independently, via Old French presbtre. The Latin word is ultimately from Greek presbyteros. This word did not refer to a religious function, the word for "priest" being Latin sacerdos, Greek hiereus.

That English should have only the single term priest for both presbyter and sacerdos came to be seen as a problem in English Bible translations. The presbyter is the minister who presides and instructs a Christian congregation, while the sacerdos is the offerer of sacrifices, in a Christian context the eucharist, performing "mediatorial offices between God and man". The feminine noun priestess is a formation coined in the 17th century, referring to female priests of the pre-Christian religions of classical antiquity and in the 20th century also came to be used in the controversies surrounding the ordination of women. However, in the case of the ordination of women in the Anglican communion, it is more common to speak of "priests" regardless of gender.