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

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "Introit":
   Agnus Dei, Alleluia, Anamnesis, Benedicite, Blessing, Canon,
   Collect, Communion, Consecration, Credo, Dismissal, Epistle,
   Fraction, Gloria, Gloria Patri, Gloria in Excelsis, Gospel,
   Gradual, Kyrie, Kyrie Eleison, Last Gospel, Lavabo, Magnificat,
   Miserere, Nunc Dimittis, Offertory, Paternoster, Pax,
   Post-Communion, Preface, Sanctus, Secreta, Te Deum, Tersanctus,
   Tract, Trisagion, Vedic hymn, alleluia, answer, anthem, antiphon,
   antiphony, canticle, chant, chorale, doxology, hallelujah, hosanna,
   hymn, hymn of praise, hymnody, hymnography, hymnology, laud,
   mantra, motet, offertory, offertory sentence, paean, psalm,
   psalmody, report, response, responsory, versicle



Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "introit":
   Negro spiritual, anthem, cantata, canticle, chorale, church music,
   doxology, gospel, gospel music, hymn, hymn-tune, hymnody,
   hymnology, mass, motet, offertory, offertory sentence, oratorio,
   paean, passion, prosodion, psalm, psalmody, recessional, requiem,
   requiem mass, sacred music, spiritual, white spiritual