TLS certificate
Also known as SSL certificate
A cryptographic credential that proves a server is who it claims to be and enables encrypted connections.
Certificates underpin HTTPS, secure mail, and VPN. They are issued by a certificate authority the client already trusts, and they expire, which is the source of most certificate incidents.
Expiry outages are entirely preventable and still common. Automated renewal and monitoring of expiry dates across every service is the fix.
Where this comes up
TLS certificate sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.