HTML is but one example of what are known as Markup Languages. The word markup comes from the copyeditor's practice of marking-up a paper with a standard set of marks that indicate how the text should be set or corrected.
There are many common markup systems available for different purposes. TeX is a markup developed by Donald Knuth which is particularly well suited to format technical papers and documents with a lot of mathematics content. SGML is a markup language that is designed to map the logical structure of a document (headers, sections, subsections, etc.). HTML is a subset of SGML, which contains specialized markups for hypertext, as well as the formatting markups that make pages appear in your browser window.