SNI
Server Name Indication: a TLS extension that lets a server host multiple SSL certificates on one IP.
SNI (Server Name Indication) is an extension to the TLS protocol that allows a client to specify which hostname it is trying to connect to at the start of the SSL/TLS handshake. Before SNI, each SSL website needed its own dedicated IP address because the server could not know which certificate to present. With SNI, a single server on a single IP can host hundreds of HTTPS websites, each with its own certificate. Virtually all modern browsers and servers support SNI. Cloudflare error 525 can sometimes be caused by SNI issues.