Explanation:
A Trojan, or Trojan horse, is a type of malware that disguises itself as a legitimate program to deceive users into installing or executing it. Once activated, it can perform various malicious actions.

Unlike viruses, Trojans do not replicate themselves. They often serve as a backdoor to systems, allowing attackers to steal data, modify files, or deploy additional malware.

Trojans are commonly delivered through phishing emails, fake software downloads, or malicious links. Users may unknowingly grant them access by trusting their legitimate appearance.

Additional Information:
Some common types of Trojans include backdoor Trojans, banking Trojans, and downloader Trojans. They can capture keystrokes, steal sensitive credentials, or allow attackers full control over an infected machine.

Prevention strategies include avoiding suspicious links, verifying software sources, and maintaining updated antivirus software. Trojans remain a persistent threat to individuals and organizations alike.

Formal Definition:
A Trojan, or Trojan horse, is malware that masquerades as a legitimate application to deceive users into executing it, often providing unauthorized access to systems or data.