introit

noun

1. A prayer, typically part of a psalm or other portion of the Bible, read or sung at the start of Mass while or immediately after the priest ascends to the altar.

   “The Glossary of Terms used by ecclesiastics in the middle ages, who describe a day by the “introit,” or commencement of the service appointed by the church to be performed thereon, and an explanation of the Canonical Hours, Watches, &c. will frequently be found useful.”

   “Adorate Dominum. The introit* and name of the third Sunday after the Epiphany. [Footnote *: Introit.—The first two or more words that form the commencement of a mass, which, from being appropriated to a certain Sunday, or other festival, give the name of such commencement or "introit" to these days.]”

   “At some time during these sacerdotal preces the choir will usually have started the sung introit, the exact time for beginning the item dependent on a number of features such as the distance from sacristy to altar.”

   “The earliest unequivocal reference to the Roman introit is from the turn-of-the-eighth-century Ordo romanus I, where the chant is described in its fully developed early medieval form. Consisting of an antiphon and psalm, it is sung during the entrance of the pope at the beginning of Mass.”

2. Any piece of choral music, especially a setting of an anthem or a psalm, sung at the opening of a church service.
3. (obsolete)The action of entering or going in; an entrance.
4. (figuratively, obsolete)An introduction.