Introit

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.

Source: wiktionary

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.]

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary