Definitions for "Malicious Code"
Any code or set of instructions specifically designed to damage computer files and/or operation. It can be as simple as locking up a program to erasing the hard drive. While some virus authors create malicious code, the bulk of virus authors just create a virus to see if they can do it, get around a back-door, or exploit a security feature, without including malicious code. One of the reasons why Morris didn't have a harsher punishment for his creation of the Internet Worm was because he did not include malicious code.
Hardware, software, of firmware that is intentionally included in a system for an unauthorized purpose; e.g. a Trojan horse.
A piece of computer code that is programmed to infect a computer system.