SNIServer Name Indication

A TLS extension that tells the server which hostname the client wants, so it can pick the right certificate.

Without SNI, you can't host multiple HTTPS sites on one IP — the server has to commit to a cert before the client says which site it wants. SNI fixes that by sending the hostname in the TLS ClientHello.

Encrypted Client Hello (ECH) is the modern successor that encrypts SNI so on-path observers (ISPs, censors) can't see which site you're visiting.