X.509
ITU-T standard for public-key certificates — the structure every TLS, code-signing and S/MIME certificate follows.
A cert is an ASN.1-DER-serialised object: subject (who the cert talks about), issuer (CA), public key, validity period, extensions (SAN, key usage, AIA, CT poison), signature from the issuer. The PEM format is base64 + -----BEGIN CERTIFICATE----- wrapping.
Original 1988 (v1) — today's v3 (RFC 5280) added extensions that enable SAN, EKU, Certificate Transparency. Despite ongoing criticism (complexity, ASN.1, RFC darkness) X.509 is so entrenched in PKI that all replacements have failed commercially.