HTML stands for HyperText Markup Language. Hypertext expresses the idea that you can click on what you are reading to jump to something else which is related. This is the same hyper in hyperspace that the characters in your favorite science-fiction story are always jumping through. The idea is to go immediately to a distant place. In the case of documents, you are going to a place which may be far away in whatever scheme is normally used to order the documents.
HTML is a way of expressing hypertext, but the idea is older. The first mention is probably Vannevar Bush's description of Memex in 1945. The term was coined by Ted Nelson in 1965, and was popularly embodied in the HyperCard application shipped with the original Apple Macintosh (from Fact-Index.com).