introit (n.)

in liturgics, "an antiphon sung as the priest approaches the altar to celebrate mass," late 15c., from Old French introite "(liturgical) introit; entrance" (14c.), from Latin (antiphona ad) introitum, from introitus "a going in, an entering, entrance; a beginning, prelude," past participle of introire "to enter," from intro- "on the inside, within" (see intro-) + ire "to go" (from PIE root *ei- "to go").