Guideword

noun

1. 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.
2. A <xref>watchword</xref>; a word or phrase that serves as one's <xref>guide</xref>
3. A word that is printed at the top or bottom of the page in a <xref>reference</xref> work, in order to help readers find a specific entry
4. Any of the words in a <xref>memo</xref> or <xref>email</xref> that define <xref>metadata</xref> regarding the message, such as sender, recipient, and subject
5. 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