Indices is one plural of index, taken from Latin. English also has indexes. Which one you see depends on the meaning and how formal the writer is. When index means a number that tracks prices, shares, or some other measure (price indices, stock indices), writers in economics, science, and maths often prefer indices. When index means the list at the back of a book, indexes is now the usual plural. In maths, index can also mean a small number written up or down beside another (a power, or a mark that says “which one”); those too are often called indices. Say IN-di-seez. Using indices for a book’s back-of-the-book lists is not wrong, but it can sound stiff.