Fail secure (fail securely) is a design principle where a system transitions to a secure state upon error or failure. Access is denied, connections are severed, and data remains protected.
The counterpart is fail open, where a failed system grants access — appropriate for fire doors that must remain open during emergencies. For IT security, fail secure is the right principle in most cases: a firewall that passes all traffic upon crashing would be disastrous. When designing physical access control, the trade-off between security and evacuation during a fire must be considered.