HTTP/2 (earlier called "HTTP/2.0") is a new version of the HTTP protocol. It is specified in a standard that is expected to be published as an RFC in 2015. Since cURL recently got HTTP/2 support the protocol can potentially be used in Tcl through TclCurl.
The HTTP/2 specification was published as RFC 7540 in May 2015.
cURL got HTTP/2 support after 7.43.0 (with the nghttp2 library).