| a series of labeled notches cut along the fore edge of a book, to indicate the divisions or sections. |
[thuhm-in-deks] 
| to provide (a book) with a thumb index. |
| thumb index n. A series of rounded indentations cut into the front edge of a book, each labeled, as with a letter, to indicate a section of the book. thumb'-in'dex (thŭm'ĭn'děks) v. |