Server Name Indication is an extension to the Transport Layer Security computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process. Wikipedia

An extension to the TLS computer networking protocol Wikipedia Disambiguation

TLS extension, where a client states the hostname at the start of the handshake so that a server can present multiple certificates on the same address/port; so it can serve multiple HTTPS sites at the same IP address with different certificates Wikidata