Introit

Definition:

(n.) A going in.
(n.) A psalm sung or chanted immediately before the collect, epistle, and gospel, and while the priest is entering within the rails of the altar.
(n.) A part of a psalm or other portion of Scripture read by the priest at Mass immediately after ascending to the altar.
(n.) An anthem or psalm sung before the Communion service.
(n.) Any composition of vocal music appropriate to the opening of church services.

Compare introit with other words:

introit vs. intromit