guideword

1. (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.Examples: "Secondly, the teacher-librarian must know how to use books herself to find information — title page, table of contents, index, guidewords, etc."

2. (chiefly business) Any of the words in a memo or email that define metadata regarding the message, such as sender, recipient, and subject.Examples: "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."