A chaplain is, traditionally, a cleric (such as a minister, priest, pastor, rabbi, purohit, or imam), or a lay representative of a religious tradition, attached to a secular institution (such as a hospital, prison, military unit, intelligence agency, embassy, school, labor union, business, police department, fire department, university, sports club), or a private chapel.

Though originally the word chaplain referred to representatives of the Christian faith, it is now also applied to people of other religions or philosophical traditions, as in the case of chaplains serving with military forces and an increasing number of chaplaincies at U.S. universities. In recent times, many lay people have received professional training in chaplaincy and are now appointed as chaplains in schools, hospitals, companies, universities, prisons and elsewhere to work alongside, or instead of, official members of the clergy. The concepts of a multi-faith team, secular, generic or humanist chaplaincy are also gaining increasing use, particularly within healthcare and educational settings.