Guideword

Noun

1. A word or phrase that serves as one's guide.
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.
3. 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.