SNI / Encrypted Client Hello
Also known as: sni, esni, ech
The field in a TLS handshake that reveals which hostname you're visiting — and the effort to encrypt it.
Even with HTTPS, the initial TLS handshake includes the Server Name Indication field in the clear so a shared server knows which certificate to present. That means your ISP or local network still sees the hostname of every site you visit.
Encrypted Client Hello (ECH) closes that leak by encrypting the SNI to a key published in DNS. It's supported by Cloudflare, Firefox and Chrome and rolling out gradually across the web.