Source : WordNet®

markup
     n 1: the amount added to the cost to determine the asking price
     2: detailed stylistic instructions for typesetting something
        that is to be printed; manual markup is usually written on
        the copy (e.g. underlining words that are to be set in
        italics)

Source : Free On-Line Dictionary of Computing

markup
     
         In computerised document preparation, a method of
        adding information to the text indicating the logical
        components of a document, or instructions for layout of the
        text on the page or other information which can be interpreted
        by some automatic system.
     
        For example, the source of {this dictionary} is marked up by
        enclosing cross-references in curly braces which are
        significant to the {World-Wide Web} server software.
     
        (1995-03-30)