HTTP

HyperText Transfer Protocol — the request–response protocol of the web: a client asks for a resource by URL with a method (GET, POST, PUT, DELETE…), and a server answers with a status code, headers and a body

Specified by the IETF (RFC 9110 for the semantics, 9112 for HTTP/1.1, 9113 for HTTP/2, 9114 for HTTP/3). HTTPS is HTTP over TLS, and is now the only version a browser treats as fully trusted. The headers are where most of the platform's behavior lives — caching, content negotiation, cookies, CORS, the content-security policy — and a REST API is HTTP's methods and status codes taken at their word.