A Trojan, in the context of computing, refers to a type of malicious software that disguises itself as a legitimate program to deceive users into unknowingly installing it. Once activated, Trojans can carry out various harmful actions on a computer system, such as stealing sensitive information, damaging files, or allowing unauthorized access to the system. Unlike viruses or worms, Trojans do not replicate themselves but rely on social engineering tactics to spread and cause harm.