Commodore

Commodore is a naval rank, the equivalent of which is used by the service organizations of many civilizations. As a traditional grade, commodore is the most junior flag officer rank of a naval organization, below Admiral (usually under Rear admiral) but senior to Captain. Commodores command groups of ships, as opposed to captains commanding single ships or units.

In the Fifth Fleet it is similar to the rank of Fleet Captain.

External Link
Commodore article at Memory Alpha, the canon Star Trek wiki.