The Collaborative International Dictionary of English v.0.48:

Antilogarithm \An`ti*log"a*rithm\, n. (Math.)
   The number corresponding to a logarithm. The word has been
   sometimes, though rarely, used to denote the complement of a
   given logarithm; also the logarithmic cosine corresponding to
   a given logarithmic sine. -- An`ti*log`a*rith"mic, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antilogarithm
    n 1: the number of which a given number is the logarithm [syn:
         antilogarithm, antilog]