Source : Webster's Revised Unabridged Dictionary (1913)

Introit \In*tro"it\, n. [L. introitus, fr. introire to go into,
   to enter; intro within + ire to go: cf. F. introit.]
   1. A going in. --Caxton.

   2. (R. C. Ch.)
      (a) A psalm sung or chanted immediately before the
          collect, epistle, and gospel, and while the priest is
          entering within the rails of the altar.
      (b) A part of a psalm or other portion of Scripture read
          by the priest at Mass immediately after ascending to
          the altar.

   3. (R. C. Ch.) An anthem or psalm sung before the Communion
      service.

   4. Any composition of vocal music appropriate to the opening
      of church services.