thumb index

US: (thum′in′deks)

WordReference Random House Unabridged Dictionary of American English © 2025

thumb′ in′dex, 

1. Printing a series of labeled notches cut along the fore edge of a book, to indicate the divisions or sections.

• 1900–05

thumb-in•dex  (thum′in′deks), v.t. 

1. Printing to provide (a book) with a thumb index.

Collins Concise English Dictionary © HarperCollins Publishers::

thumb index noun

1. a series of indentations cut into the fore-edge of a book to facilitate quick reference

verb thumb-index

1. (transitive) to furnish with a thumb index