What's the difference between i.e. and e.g.?
c.1200, from Old French letanie and directly from Medieval Latin letania, Late Latin litania (cf. Spanish letania, Italian litania), from Greek litaneia "litany, an entreating," from lite "prayer, supplication, entreaty," of unknown origin. From notion of monotonous enumeration of petitions in Christian prayer services came generalized sense of "repeated series," early 19c., borrowed from French.
For those who know the Greek words, a litany is a series of prayers, a liturgy is a canon of public service; the latter in practice includes prayer, but does not say so. [Fowler]
In many religions, a ritual repetition of prayers. Usually a clergyman or singer chants a prayer, and the congregation makes a response, such as “Lord, have mercy.”