Definition of guideword:
(n) :
(publishing) 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.
(n) :
(chiefly business) Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject.
(n) :
A watchword; a word or phrase that serves as one's guide.