a contagious viral disease of cows that causes pustules on the udders: people inoculated with a vaccine containing the virus of cowpox become temporarily immune to smallpox
See cowpox in American Heritage Dictionary 4
noun
A mild contagious skin disease of cattle, usually affecting the udder, that is caused by a virus and characterized by the eruption of a pustular rash. When the virus is transmitted to humans, as by vaccination, it can confer immunity to smallpox. Also called vaccinia.