[durazabu]

The index and the staging area are not the same thing. If you define staging area as what will be committed, then the staging area is only one part of the index. The index also records untracked files and conflicts for example.

https://git-scm.com/docs/index-format

[rmwaite]

I've always understood the index as being the staging area and then you stage things in that staging area. Then those things become staged (also known as cached) in the index. I'll admit I've never quite understood why "cached" and "staged" are used interchangeably (or at least it seems that way).