Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

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

Source : WordNet®

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