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 Ted Nelson in 1965, and was
popularly embodied in the HyperCard application shipped with the
original Apple Macintosh 
(see http://www.fact-index.com/h/hy/hypertext.html).