Glossary / SSL certificate

What is an SSL certificate

An SSL certificate is a digital file that proves a website really belongs to its owner and lets the browser and server encrypt their connection. Without it, data travels as plain text and browsers flag the site as not secure. The certificate is what turns on the padlock and the https address.

Technically the certificate is a file with a public key and domain details, signed by a certificate authority (CA). When you open a site, the server presents its certificate and the browser checks the signature and expiry date. If everything matches, the connection is encrypted.

A certificate is tied to a specific domain or subdomain. One certificate can cover several names, or a whole subdomain level if it is a wildcard. Its lifetime is limited: for free certificates it is usually 90 days, so renewal is automated.

For visitors the certificate is about trust: without it the browser throws a warning and Google ranks the site lower. For the owner it is the baseline requirement for forms, payments, and account logins.

Example

Subject: CN=example.com  Issuer: R11 (Let's Encrypt)  Expires: 2026-10-14

Common mistakes

• Assuming the certificate makes the whole site secure. It only encrypts the channel, it does not fix vulnerabilities in the site itself.
• Forgetting about auto-renewal and ending up with an expired certificate that blocks the site.
• Installing it only on www or only on the bare domain and leaving the other version uncovered.
• Confusing paid and free certificates on encryption strength: the encryption is identical, only validation and warranty differ.

Related terms

• TLS What is TLS
• HTTPS What is HTTPS
• wildcard certificate What is a wildcard certificate
• Let's Encrypt What is Let's Encrypt
• certificate chain What is a certificate chain