A type of malicious computer program, similar to a virus, that pretends to be a normal program but causes damage such as erasing a hard disk drive when the unsuspecting user executes it. A virus, on the other hand, typically spreads unseen, hides in inaccessible areas of the computer, and causes its damage spontaneously at predetermined times or dates. Most email viruses are in fact Trojans, since they rely on the user opening them to execute.