A word that is printed at the top or bottom of the page in a reference work, in order to help readers find a specific entry.
A watchword; a word or phrase that serves as one's guide.
Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject.
A word used as the title of a section, particularly in a dictionary, encyclopedia, or thesaurus.
Any word which may be modified by an adjunct.