[Spivak]

I will say that, at least for me, indices and indexes aren’t the same word and mean two different things. This has made me realize that I’ve just naturally made this distinction without really knowing it.

* In a loop i,j,k are the indices. Matrices have indices too.

* Databases have indexes. The things at the back of books are indexes.

Indices are things that represent a position in something. Indexes are the things — the literal “objects” or mappings or trackers.