The Collaborative International Dictionary of English v.0.48:

Postcommunion \Post`com*mun"ion\, n. [Pref. post- + communion.]
   1. (Ch. of Eng. & Prot. Epis. Ch.) The concluding portion of
      the communion service.
      [1913 Webster]

   2. (R. C. Ch.) A prayer or prayers which the priest says at
      Mass, after the ablutions.
      [1913 Webster]