In computing, an HTML element (instance) in terms of SGML is the complete sequence of a start tag (with attributes and their values), any embedded HTML content and the end tag. A special case is empty elements that don't have contents and end tags. Due to the constraints of the DTDs, various parts, including start and end tag, may be omitted in HTML, but not XHTML.
It is often misnamed "tag", in a ugly kind of pars-pro-toto.