TLS
Also known as: transport layer security, ssl
The cryptographic protocol that secures HTTPS and most modern network traffic.
TLS (formerly SSL) sits between TCP and application protocols like HTTP. It authenticates the server via certificates, negotiates encryption keys and then encrypts everything that follows. Modern versions (TLS 1.3) are faster and drop legacy cipher suites.
TLS is what makes the address-bar padlock meaningful: without it, any network between you and the site can read or alter traffic. It doesn't authenticate you to the server or vouch for what the server does with your data.