To help sargun out more, Hopper was an O-7, where naval officer's ranks go from O-1 (ensign) to O-10 (admiral). This rank -- along with O-8 (rear admiral (upper half)) -- was previously called "commodore," and in some other countries still is, as in Commodore Matthew Perry.
e: What was previously commodore was split into the current two rear admiral ranks.
Thats because the US Navy has two rear admiral ranks. The lower rear admiral is a 1 star officer and is equivalent to commodores in other navys. The upper rear admiral is a 2 star officer is equivalent to "normal" rear admirals in most other navies.
[1] https://en.wikipedia.org/wiki/Rear_admiral