The Collaborative International Dictionary of English v.0.48:

Cowpox \Cow"pox`\ (-p[o^]ks`), n. (Med.)
   A pustular eruptive disease of the cow, which, when
   communicated to the human system, as by vaccination, protects
   from the smallpox; vaccinia; -- called also kinepox,
   cowpock, and kinepock. --Dunglison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cowpox
    n 1: a viral disease of cattle causing a mild skin disease
         affecting the udder; formerly used to inoculate humans
         against smallpox [syn: cowpox, vaccinia]