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index (in a book)
この辞書の索引を使えば単語が早く見つかるよ。
You can find words quickly using this dictionary's index.
本の巻末には索引がついているのが便利だね。
It's convenient that there's an index at the end of the book.
索引を見て必要なページをすぐに開くことができた。
I could immediately open the needed page by looking at the index.