SSL / TLS
The protocol that encrypts traffic between client and server — what turns HTTP into HTTPS.
SSL is the old name (last version 3.0, 1996). TLS is the successor. When people say "SSL certificate" today, they almost always mean TLS — the name just stuck.
TLS 1.3 (2018) is current. Earlier versions (1.0, 1.1) are considered insecure and should be disabled. Certificates are free from Let's Encrypt.