Definition of introits:
(n) :
(Roman Catholicism, music) 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.
(n) :
(music, Christianity, chiefly Protestantism) Any piece of choral music, especially a setting of an anthem or a psalm, sung at the opening of a church service.
(n) :
(obsolete) The action of entering or going in; an entrance.
(n) :
(obsolete, figuratively) An introduction.