Backdoor
A hidden way into a system that bypasses normal authentication and security checks.
A backdoor↗ is a hidden way into a system that bypasses normal security checks such as authentication↗ and authorization↗. It lets an attacker come back in secretly, whenever they want, even if the "front door" is locked.
Backdoors take different forms: a hidden access point planted by malware↗ (such as a trojan↗), a mechanism an attacker leaves behind for persistence after a compromise, or a flaw arising from careless or poor software design. Sometimes access left in place by developers for maintenance, and never cleaned up, unintentionally becomes a backdoor.
The existence of a backdoor means a system is at continuous risk, not a one-time incident, so simply removing the visible malware from a compromised system is not enough. Every persistence mechanism, added account, and altered configuration must be fully identified and removed, and often the safest approach is to rebuild the system from a clean source entirely.