1. A poxvirus (species Vaccinia virus of the genus Orthopoxvirus) that differs from but is closely related to the viruses causing smallpox and cowpox and that includes a strain of uncertain natural origin used in making vaccines against smallpox — called also vaccinia virus.
2. A reaction to smallpox vaccine prepared from live vaccinia virus that may involve a rash, fever, headache, and body pain SYN. cow-pox.