TLS
The protocol that encrypts HTTP and other connections — what the lock icon in browsers actually means.
Definition
Transport Layer Security encrypts data between client and server. HTTPS is HTTP over TLS. Certificates issued by trusted CAs (Let's Encrypt is the default for small sites) authenticate the server. Modern web traffic, MCP HTTP transports, webhook deliveries, and most API calls all require TLS; cleartext HTTP is rejected by most browsers and CDNs.