A Certificate Revocation List (CRL) is a list published by a certificate authority (CA) that contains digital certificates that have been revoked before their scheduled expiration date. This can happen for various reasons, such as a compromised private key or a change in the status of the certificate holder. The CRL helps ensure that users and systems can verify the validity of certificates they encounter. CRLs are essential for maintaining the security of digital communications, as they prevent the use of invalid or untrustworthy certificates. When a system encounters a certificate, it can check the CRL to determine if the certificate is still valid. This process is crucial for maintaining trust in systems that rely on public key infrastructure (PKI) and secure communications.