https://html.spec.whatwg.org/multipage/introduction.html#is-this-html5?
12 August 2016
1.2 Is this HTML5?
This section is non-normative.
In short: Yes.
In more length: The term "HTML5" is widely used as a buzzword to refer to modern Web technologies, many of which (though by no means all) are developed at the WHATWG. This document is one such; others are available from the WHATWG specification index.
Although we have asked them to stop doing so, the W3C also republishes some parts of this specification as separate documents.