Source : Webster's Revised Unabridged Dictionary (1913)

Commodore \Com"mo*dore`\, n. [Prob. a corruption of commander,
   or Sp. comendador a knight of a military order who holds a
   commandery; also a superior of a monastery, fr. LL.
   commendare to command. Cf. {Commend}, {Command},
   {Commander}.]
   1. (U. S. Navy) An officer who ranks next above a captain;
      sometimes, by courtesy, the senior captain of a squadron.
      The rank of commodore corresponds with that of brigadier
      general in the army.

   2. (British Navy) A captain commanding a squadron, or a
      division of a fleet, or having the temporary rank of rear
      admiral.

   3. A title given by courtesy to the senior captain of a line
      of merchant vessels, and also to the chief officer of a
      yachting or rowing club.

   4. A familiar for the flagship, or for the principal vessel
      of a squadron or fleet.

Source : WordNet®

commodore
     n : a commissioned naval officer who ranks above a captain and
         below a rear admiral; the lowest grade of admiral