The Collaborative International Dictionary of English v.0.48:

Offertory \Of"fer*to*ry\, n.; pl. Offertories . [L.
   offertorium the place to which offerings were brought, in LL.
   offertory: cf. F. offertoire.]
   1. The act of offering, or the thing offered. [Obs. or R.]
      --Bacon. --Bp. Fell.
      [1913 Webster]

   2. (R. C. Ch.)
      (a) An anthem chanted, or a voluntary played on the organ,
          during the offering and first part of the Mass.
      (b) That part of the Mass which the priest reads before
          uncovering the chalice to offer up the elements for
          consecration.
      (c) The oblation of the elements.
          [1913 Webster]

   3. (Ch. of Eng. & Prot. Epis. Ch.)
      (a) The Scripture sentences said or sung during the
          collection of the offerings.
      (b) The offerings themselves.
          [1913 Webster]

WordNet (r) 3.0 (2006):

offertory
    n 1: the offerings of the congregation at a religious service
    2: the part of the Eucharist when bread and wine are offered to
       God

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "offertory":
   Agnus Dei, Benedicite, Gloria, Gloria Patri, Gloria in Excelsis,
   Introit, Magnificat, Miserere, Negro spiritual, Nunc Dimittis,
   Te Deum, Trisagion, Vedic hymn, alleluia, alms, alms fee, answer,
   anthem, antiphon, antiphony, burnt offering, cantata, canticle,
   chant, charity, chorale, church music, collection, contribution,
   dole, donation, donative, doxology, drink offering,
   ex voto offering, gospel, gospel music, hallelujah, handout,
   heave offering, hecatomb, holocaust, hosanna, human sacrifice,
   hymn, hymn of praise, hymn-tune, hymnody, hymnography, hymnology,
   immolation, incense, infanticide, introit, laud, libation,
   mactation, mantra, mass, motet, oblation, offering,
   offertory sentence, oratorio, paean, passion, peace offering,
   piacular offering, pittance, prosodion, psalm, psalmody,
   recessional, report, requiem, requiem mass, response, responsory,
   sacramental offering, sacred music, sacrifice, scapegoat,
   self-immolation, self-sacrifice, spiritual, subscription, suttee,
   sutteeism, thank offering, tithe, versicle, votive offering,
   white spiritual, whole offering