The Collaborative International Dictionary of English v.0.48:

Concordat \Con*cor"dat\, n. [F. concordat, L. concordato, prop.
   p. p. of concordare. See Concord.]
   1. A compact, covenant, or agreement concerning anything.
      [1913 Webster]

   2. An agreement made between the pope and a sovereign or
      government for the regulation of ecclesiastical matters
      with which both are concerned; as, the concordat between
      Pope Pius VII and Bonaparte in 1801. --Hook.
      [1913 Webster]

WordNet (r) 3.0 (2006):

concordat
    n 1: a signed written agreement between two or more parties
         (nations) to perform some action [syn: covenant,
         compact, concordat]