The Collaborative International Dictionary of English v.0.48:

Pax \Pax\, n. [L. pax peace. See Peace.]
   1. (Eccl.) The kiss of peace; also, the embrace in the
      sanctuary now substituted for it at High Mass in Roman
      Catholic churches.
      [1913 Webster]

   2. (R. C. Ch.) A tablet or board, on which is a
      representation of Christ, of the Virgin Mary, or of some
      saint and which, in the Mass, was kissed by the priest and
      then by the people, in medi[ae]val times; an osculatory.
      It is still used in communities, confraternities, etc.
      [1913 Webster]

            Kiss the pax, and be quiet like your neighbors.
                                                  --Chapman.
      [1913 Webster]

   3. Friendship, or a friend; -- esp. in the phrases to make
      pax with, to make friends with, to be good pax, to be good
      friends; also, truce; -- used esp. interjectionally. [Eng.
      Schoolboy Slang]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

pax
    n 1: (Roman Catholic Church) a greeting signifying Christian
         love for those assisting at the Eucharist [syn: pax,
         kiss of peace]