1) Book design 2) Notches in book edges 3) Reference help 
A thumb index, also called a cut-in index, is a round cut-out in the pages of dictionaries, encyclopedias, or other sectioned, often alphabetic, reference works like Bibles, used to locate entries starting at a particular letter or section. The individual notches are called thumb cuts. ...
[n] - one of a series of rounded notches in the fore edge of a book to indicate sections
noun one of a series of rounded notches in the fore edge of a book to indicate sections