The Collaborative International Dictionary of English v.0.48:

Catchword \Catch"word`\, n.
   1. Among theatrical performers, the last word of the
      preceding speaker, which reminds one that he is to speak
      next; cue.
      [1913 Webster]

   2. (Print.) The first word of any page of a book after the
      first, inserted at the right hand bottom corner of the
      preceding page for the assistance of the reader. It is
      seldom used in modern printing.
      [1913 Webster]

   3. A word or phrase caught up and repeated for effect; as,
      the catchword of a political party, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

catchword
    n 1: a favorite saying of a sect or political group [syn:
         motto, slogan, catchword, shibboleth]
    2: a word printed at the top of the page of a dictionary or
       other reference book to indicate the first or last item on
       that page [syn: guide word, guideword, catchword]