Describes the way in which the tags on a web page relate to the content they surround. Tags must make sense for the information inside them. For example, if you have a h2 heading on a page, you must also have a h1 heading because the h2 heading must be a sub-heading of something. Or, if you use a definition list tag the text contained must contain a word or phrase and a definition.

with regard to meaning; "semantically empty messages"