The Collaborative International Dictionary of English v.0.48:

Cata \Cat"a\ [Gr. kata`.]
   The Latin and English form of a Greek preposition, used as a
   prefix to signify down, downward, under, against, contrary or
   opposed to, wholly, completely; as in cataclysm, catarrh. It
   sometimes drops the final vowel, as in catoptric; and is
   sometimes changed to cath, as in cathartic, catholic.
   [1913 Webster]