TLS Transport Layer Security
Crypto protocol providing confidentiality + integrity + authentication on TCP. HTTPS, IMAPS, SMTPS, gRPC and many more all run over TLS.
Evolution: SSL 1.0 (Netscape, never published), SSL 2.0 (1995, killed), SSL 3.0 (1996, POODLE 2014), TLS 1.0 (RFC 2246, 1999), TLS 1.2 (2008, dominant until the 2020s), TLS 1.3 (RFC 8446, 2018, fewer round-trips, modern cipher suites). Cert validation requires a CA. Let's Encrypt (2016) made free certificates universal. mTLS adds client certs for mutual authentication.