Typically unauthorized hidden software or hardware mechanism used to circumvent security controls.
An undocumented way of gaining access to a computer system. A backdoor is a potential security risk.
SOURCE: SP 800-82
A malicious program that listens for commands on a certain Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port.
SOURCE: GUIDE TO MALWARE INCIDENT PREVENTION AND HANDLING FOR DESKTOPS AND LAPTOPS