guideword

noun

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

   “Naturalness is perhaps the best guideword to keep in mind.”

   “Heidegger accompanies this meditation, however, with a second, more overtly "philosophical" or "thinking" reflection on the "guideword" Das Wesen der Sprache: -- die Sprache des Wesens ("The essence of language: -- the language of essence").”

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.”

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. (uncommon)Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject.

   “One purpose of a traditional memo or e-mail is to provide essential information quickly and easily, so there is no restriction, other than appearance or practicality, on the number or variety of guidewords you include.”