SSL certificate
A cryptographic credential that authenticates a domain and enables HTTPS. It has an expiry date; let it lapse and the site breaks.
An SSL certificate (more accurately a TLS certificate — SSL is the older protocol that TLS replaced, but the colloquial name stuck) is a cryptographic credential issued by a certificate authority that authenticates a domain to visiting browsers and enables HTTPS encryption.
Certificates have an expiry date, typically 90 days for Let’s Encrypt or up to 13 months for most paid issuers. When the expiry passes, browsers refuse to load the site without a hard warning, which functionally takes the site down for the public internet. Certificate expiry is one of the most common avoidable outage causes; the fix is automated renewal — and a monitor that checks the certificate’s remaining validity weeks before expiry.