noun
(cryptography) Any word used as the key to a code.
(information science) Any word used in a reference work to link to other words or other information.
(information science) Any important word in a text or document, which may be linked to other words or other information, or listed in the metadata for searches to find.
(programming) A reserved word used to identify a specific command, function, etc.
(linguistics) Any word that occurs in a text more often than normal.
verb
(transitive) To tag with keywords, as for example to facilitate searching.