A chaplain is a member of the clergy who works in a lay, or non-religious, institution, such as an army, a prison, a hospital, or a university. Other members of the clergy typically work in a church or mission setting. Lay chaplains, who minister but are not ordained, are becoming increasingly common. The term lay minister or lay clergy may also be used for chaplains who are ordained to indicate that they work outside of a religious institution; however, this use of the term is no longer common.
Though usually associated with Roman Catholicism or other Christian denominations, the idea of a chaplain is not limited to Christianity. There are Muslim, Jewish, and Buddhist equivalents, for example, in the Armed Forces of both Great Britain and the United States. The practice of priests serving outside of religious institutions has existed since ancient times, centuries before the time of Christ.
In Roman Catholicism, a chaplain was originally a relatively low-ranked member of the clergy who served a higher ranked member, a bishop, and performed mass on certain occasions. Chaplains of this definition still serve the pope. In medieval Europe, clergymen were among the only literate members of society, and therefore served in royal or noble courts as both religious advisers and secretarial staff — hence the term clerical, literally "of the clergy," can be used to describe office work.
A form of the military chaplain has also existed since the medieval period, when clergy members served on England's military vessels. Militaries around the world have chaplains, though their rank and qualifications differ widely. In some countries, the military chaplain is a soldier with theological training, while in others, the chaplain is a civilian, though an ordained priest. According to the Geneva Conventions, a military chaplain is a noncombatant, not authorized to engage in combat. In addition, a chaplain may not be held as a Prisoner of War, but may be retained by the enemy for the purpose of ministering to Prisoners of War.