What is the origin of the word commodore?

It is a rank between Captain and Rear Admiral in the navy. A Captain is is command of one ship (his/her own) and a Commodore is in charge of more than one ship. The RAF has an Air-Commodore which is a rank above a Group Captain and below that of an Air-Marshal. The equivalent rank in the army is a Brigadier who is in charge of a brigade of men 3,000 to 5,000 in total. It is from the French term Commandeur which was one of the highest ranks in Knighthood.