guideword

noun4

1. •
   A word or phrase that serves as one's guide.

   • Naturalness is perhaps the best guideword to keep in mind. — 1947
2. •
   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.

   • Secondly, the teacher-librarian must know how to use books herself to find information — title page, table of contents, index, guidewords, etc. — 1936
3. •
   (historical) A word under the right-hand side of the last line on a book page that repeats the first word from the following page.
4. •
   Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject.