Acronym. HyperText Markup Language.
HTML 5 (formerly and commonly spelled HTML5) is a software solution stack that defines the properties and behaviors of web page content by implementing a markup based pattern to it. HTML 5 is the fifth and current major version of the HTML standard, and subsumes HTML 4 and XHTML.
HTML 5 was first released in public-facing form on January 22, 2008, with a major update and "W3C Recommendation" status in October 2014. Its goals are to improve the language with support for the latest multimedia and other new features; to keep the language both easily readable by humans and consistently understood by computers and devices such as Web browsers, parsers, etc., without XHTML's rigidity; and to retain backward compatibility with older software.
HTML 5 includes detailed processing models to encourage more interoperable implementations; it extends, improves and rationalizes the markup available for documents, and introduces markup and application programming interfaces (APIs) for complex web applications. For the same reasons, HTML 5 is also a candidate for cross-platform mobile applications, because it includes features designed with low-powered devices in mind.