Any word which may be modified by an adjunct.
A word used as the title of a section, particularly in a dictionary, encyclopedia, or thesaurus.
Any word used in a reference work to link to other words or other information.
A reserved word used to identify a specific command, function, etc.
Any important word in a text or document, which may be linked to other words or other information, or at least merely listed in the metadata for searches to find.
Any word used as the key to a code.
Any word that occurs in a text more often than normal.
To tag with keywords, as for example to facilitate searching.