A backdoor is a type of malicious software or a security vulnerability that provides unauthorized access to a computer system, application, or network. It is designed to bypass normal authentication and security measures, allowing attackers to gain remote access and control over the system without the knowledge of legitimate users.

Backdoors are often created and used by hackers and cybercriminals to gain persistent access to a compromised system, enabling them to steal sensitive information, install additional malware, or carry out other malicious activities. Backdoors can be discreetly inserted into software, operating systems, or network devices, making them difficult to detect.