The term third order signifies, in general, lay members of Christian religious orders, who do not necessarily live in a religious community such as a monastery or a nunnery, and yet can claim to wear the religious habit and participate in the good works of a great order. Roman Catholicism, Lutheranism and Anglicanism all recognize third orders.

Third orders were a 12th-century adaptation of the medieval monastic confraternities. Members of third orders are known as tertiaries (Latin tertiarii, from tertius, "third"). In some cases, they may belong to a religious institute (a "congregation") that is called a "third order regular".

Roman Catholic canon law states:

"Associations whose members share in the spirit of some religious institute while in secular life, lead an apostolic life, and strive for Christian perfection under the higher direction of the same institute are called third orders or some other appropriate name."