Dictionary.net

a local infection induced in humans by innoculation with the virus causing cowpox in order to confer resistance to smallpox; normally lasts three weeks and leaves a pitted scar

1. Cow-pox, a contagious eruptive disease occurring in cattle; it is similar in its lesions to smallpox in man, but much milder; inoculation of man with the virus of this disease confers more or less lasting immunity against smallpox. 2. A disease, usually local and limited to the site of inoculation, induced in man by inoculation with the virus of cowpox (vaccination). On about the third day after vaccination papules form at the site of inoculation which become transformed into umbilicated vesicles and later pustules; they then dry up and the scab falls off about the 21st day, leaving a pitted scar; in some cases there are more or less marked constitutional disturbances.

See Vaccinia.