Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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