I believe the Answer is Thesaurus.
If I'm wrong then I'm very sorry.
A in·dex- <em>an alphabetical list of names, subjects, etc., with references to the places where they occur, typically found at the end of a book.</em>
A glos·sa·ry-<em> an alphabetical list of terms or words found in or relating to a specific subject, text, or dialect, with explanations; a brief dictionary.</em>
A the·sau·rus- <em>a book that lists words in groups of synonyms and related concepts.</em>
A dic·tion·ar·y- <em>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.</em>
So i believe its Thesaurus
