Online Certificate Status Protocol (OCSP) is used for obtaining the status of X.509 digital certs.
The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate. It is described in RFC 6960 and is on the Internet standards track. It was created as an alternative to certificate revocation lists (CRL), specifically addressing certain problems associated with using CRLs in a public key infrastructure (PKI). Messages communicated via OCSP are encoded in ASN.1 and are usually communicated over HTTP. The "request/response" nature of these messages leads to OCSP servers being termed OCSP responders.
Some web browsers (Firefox) use OCSP to validate HTTPS certificates, while others have disabled it. Most OCSP revocation statuses on the Internet disappear soon after certificate expiration.