A Certificate Revocation List (CRL) is a list of digital certificates that have been revoked and are no longer valid. When a certificate is issued to an entity, it is valid for a specific period. However, due to various reasons such as security breaches, changes in user status, or certificate expiration, certificates may need to be revoked before their scheduled expiration date. A CRL is a repository of such revoked certificates, which helps to prevent their use in secure communication.