Markup refers to annotations to text that are not part of the core text as such, but add formatting or meaning. The most familiar is likely to be HTML which has tags such as <em> to mean "display in italics". Semantic markup adds explicit meanings, for example microformats are HTML classes that can enable the text to be interpretted by a suitable program, for example:
<span class="givenname">Alan<\span> <span class="familyname">Dix<\span>.
Used in Chap. 17: pages 250, 258, 259, 266
Links:
microformats.org: Information about common microformats- schema.org: Schema.org – industry standard vocabulary to use on RDFa, Microdata and JSON-LD.