HTTPS
Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP) for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protocol is encrypted using Transport Layer Security (TLS), or, formerly, its predecessor, Secure Sockets Layer (SSL). The protocol is therefore also often referred to as HTTP over TLS, or HTTP over SSL.
HTTP/1.1 Upgrade header
One use is to begin a request on the normal HTTP port but switch to Transport Layer Security (TLS). In practice such use is rare, with HTTPS being a far more common way to initiate encrypted HTTP.