Virus

- A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes.
- A computer program that can copy itself and infect a computer without the permission or knowledge of the owner.(1)
- A software program capable of reproducing itself and usually capable of causing great harm to files or other 
  programs on the same computer.(2)

Sources (1) - http://en.wikipedia.org/wiki/Computer_virus (2) - http://wordnetweb.princeton.edu/perl/webwn?s=computer%20virus