Word Index Found at the Back of Books
Puzzle 17 of National Crossword Day Hard Pack
Question is: Word Index Found at the Back of Books
and the answer is: GLOSSARY
Word Meaning:
GLOSSARY — A list of difficult words or specialized terms used in a particular book or document, or in a particular domain of knowledge, with their definitions; a list of glosses (explanatory annotations).
Definitions from Wiktionary.