A Virus is an unwanted and often malicious and destructive program.
What is a Virus?
(definition from Wikipedia.org)
A computer virus is a self-replicating computer program written to alter the way a computer operates, without the permission or knowledge of the user. Though the term is commonly used to refer to a range of malware, a true virus must replicate itself, and must execute itself. The later criteria is often met by a virus which replaces existing executable files with a virus-infected copy. While viruses can be intentionally destructive—destroying data, for example—some viruses are benign or merely annoying.