SSL certificate
Also known as: TLS, HTTPS
An SSL certificate enables encrypted HTTPS connections, protecting data in transit between a visitor's browser and the website.
In full
An SSL certificate, more accurately a TLS certificate, allows a website to be served over HTTPS, encrypting traffic between the visitor's browser and the server so it cannot be read or altered in transit. It also verifies that the site is served by whoever controls the domain. Browsers display warnings on sites without one, and modern browser features increasingly require a secure context. Certificates are now generally free and automatically renewed through providers such as Let's Encrypt, which removed the cost argument that once justified unencrypted sites.
Why it matters
An expired certificate produces a full-page browser warning that stops most visitors. It is among the most damaging routine failures a website can have, and among the easiest to monitor for.