A markup language is a modern system for annotating text in a fashion that is syntactically distinguishable from that text. This idea and terminology originated from the "marking up" of manuscripts, i.e. the revision instructions by editors (traditionally written with a blue pencil on author's manuscript). The markup is usually omitted from the version of the text which get finally rendered or printed.
There are heavy and lightweight markup languages. To the heavy ones are considered HTML, XML and Latex as they heavily incorporate syntax for presentation into the actual content and they are difficult to read. In contrast a lightweight markup language has a simple syntax, is designed to be easy for humans as well as machines to write and read even in its raw form.