What Is an SSL Certificate?
An SSL certificate, more accurately a TLS certificate today, binds a public key to one or more domain names so browsers can establish encrypted HTTPS connections.
Updated 2026-08-08What it contains
A certificate includes subject names, issuer information, validity dates, a public key and signatures that chain back to a trusted certificate authority.
Browsers compare the requested hostname with the certificate names and check whether the certificate is currently valid.
Why expiration matters
Expired certificates cause browser warnings or connection failures. Renewal keeps domain validation current and limits the lifetime of old keys or issuance mistakes.
When to check it
Check certificates before launches, after DNS or CDN changes, during incident response and when monitoring renewal windows.
Quick answers
Can one certificate cover many hostnames?
Yes. Subject Alternative Names can list multiple names, and wildcard certificates can cover one label level under a domain.