The Collaborative International Dictionary of English v.0.48:

Introit \In*tro"it\, n. [L. introitus, fr. introire to go into,
   to enter; intro within + ire to go: cf. F. introit.]
   [1913 Webster]
   1. A going in. --Caxton.
      [1913 Webster]

   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.
          [1913 Webster]

   3. (R. C. Ch.) An anthem or psalm sung before the Communion
      service.
      [1913 Webster]

   4. Any composition of vocal music appropriate to the opening
      of church services.
      [1913 Webster]

WordNet (r) 3.0 (2006):

introit
    n 1: a composition of vocal music that is appropriate for
         opening church services