catchword

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

• noun A well-known word or phrase, especially one that exemplifies a notion, class, or quality.
• noun A guideword.
• noun The first word of a page printed in the bottom right-hand corner of the preceding page.

from The Century Dictionary.

• noun In old writing and printing, a word of the text standing by itself in the right-hand corner of the bottom of a page, the same as the first word of the next page, to mark the connection or proper sequence.
• noun In the drama, the last word of a speaker, which serves to remind the one who is to follow him of what he is to say; a cue.
• noun A word caught up and repeated for effect; a taking word or phrase used as a partizan cry or shibboleth: as, the catchword of a political party.

from the GNU version of the Collaborative International Dictionary of English.

• noun Among theatrical performers, the last word of the preceding speaker, which reminds one that he is to speak next; cue.
• noun (Print.) The first word of any page of a book after the first, inserted at the right hand bottom corner of the preceding page for the assistance of the reader. It is seldom used in modern printing.
• noun A word or phrase caught up and repeated for effect

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

• noun A word under the right-hand side of the last line on a book page that repeats the first word on the following page.
• noun A word or expression repeated until it becomes representative of a party, school, business, or point of view.

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

• noun a favorite saying of a sect or political group
• 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