Definition of "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.]

Harris Nicolas, “Preface”, in Dionysius Lardner, editor, The Chronology of History. […] (The Cabinet Cyclopædia; XLIV), London: […] Longman, Rees, Orme, Brown, Green, & Longman, […]; and John Taylor, […], page xvii

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.]

1833, [Nicholas] Harris Nicolas, “A Glossary of Dates; […]”, in Dionysius Lardner, editor, The Chronology of History. […] (The Cabinet Cyclopædia; XLIV), London: […] Longman, Rees, Orme, Brown, Green, & Longman, […]; and John Taylor, […], page 110, column 1

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.

1982, Andrew Hughes, “Mass”, in Medieval Manuscripts for Mass and Office: A Guide to Their Organization and Terminology, Toronto, Ont., Buffalo, N.Y.: University of Toronto Press, published 2004, page 84

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.

2000, James [W.] McKinnon, “The Introit”, in The Advent Project: The Later-seventh-century Creation of the Roman Mass Proper, Berkeley, Los Angeles, Calif., London: University of California Press, page 195