guideword

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A word or term that appears at the top of each page or column in a reference book, such as a dictionary, to indicate the first or last entry on the page.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A watchword; a word or phrase that serves as one's guide
• noun 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
• noun Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a word printed at the top of the page of a dictionary or other reference book to indicate the first or last item on that page