The Collaborative International Dictionary of English v.0.48:

Caper \Ca"per\, n. [F. c[^a]pre, fr. L. capparis, Gr. ?; cf. Ar.
   & Per. al-kabar.]
   1. The pungent grayish green flower bud of the European and
      Oriental caper (Capparis spinosa), much used for
      pickles.
      [1913 Webster]

   2. (Bot.) A plant of the genus Capparis; -- called also
      caper bush, caper tree.
      [1913 Webster]

   Note: The Capparis spinosa is a low prickly shrub of the
         Mediterranean coasts, with trailing branches and
         brilliant flowers; -- cultivated in the south of Europe
         for its buds. The Capparis sodada is an almost
         leafless spiny shrub of central Africa (Soudan),
         Arabia, and southern India, with edible berries.
         [1913 Webster]

   Bean caper. See Bran caper, in the Vocabulary.

   Caper sauce, a kind of sauce or catchup made of capers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caper sauce
    n 1: allemande sauce with capers