I believe the Answer is Thesaurus.
If I'm wrong then I'm very sorry.
A in·dex- an alphabetical list of names, subjects, etc., with references to the places where they occur, typically found at the end of a book.
A glos·sa·ry- an alphabetical list of terms or words found in or relating to a specific subject, text, or dialect, with explanations; a brief dictionary.
A the·sau·rus- a book that lists words in groups of synonyms and related concepts.
A dic·tion·ar·y- a book or electronic resource that lists the words of a language (typically in alphabetical order) and gives their meaning, or gives the equivalent words in a different language, often also providing information about pronunciation, origin, and usage.
So i believe its Thesaurus
