The word markup was originally used to describe annotation or other marks within a text intended to instruct a compositor or typist how a particular passage should be printed or laid out. Examples, familiar to proofreaders and others, include wavy underlining to indicate boldface, special symbols for passages to be omited or printed in a particular font, and so forth. As the production of texts was automated, the term was extended to cover all sorts of special “markup codes” inserted into electronic texts to govern formatting, printing, or other processing.
Generalizing from that sene [sic], we define markup, or (synonymously) encoding, as any means of making explicit an interpretation of a text. At a banal level, all printed texts are encoded in this sense: punctuation marks, use of capitalization, disposition of letters around the page, even the spaces between words, might all be regarded as a kind of markup, the function of which is to help the human reader determine where one word ends and another begins, or how to identify gross structural features such as headings, and syntactic units such as dependent clauses or sentences. Encoding a text for a computer processing is in principle, like transcribing a manuscript from scriptio continua, a process of making explicit what is conjectural or implicit. It is a process of directing the user as to how the content of the text should be interpreted.