Worm
1. See malicious code.
2. Programs that propagate from computer to computer on a network, without necessarily modifying other programs on the target machines.
Source:
1. CNSS Instruction No. 4009
2. Garfinkel, S. & Spafford, G. (2003) Practical UNIX and Internet Security (3rd ed.). Sebastopol, CA: O'Reilly & Associates, Inc.