Key rotation is the practice of systematically replacing cryptographic keys with new ones on a scheduled basis, or in response to compromise. A key (API key, encryption key, TLS cert, database password) has a lifecycle: creation, active use, rotation, retirement. Each rotation creates a new key, starts using it, and eventually disables the old key. The process must be automated to avoid human error. Tools (Vault, AWS KMS, cert-manager) manage the entire lifecycle—generation, distribution, rotation, audit logging, and emergency procedures.