Source : Webster's Revised Unabridged Dictionary (1913)

Rear \Rear\, a.
   Being behind, or in the hindmost part; hindmost; as, the rear
   rank of a company.

   {Rear admiral}, an officer in the navy, next in rank below a
      vice admiral, and above a commodore. See {Admiral}.

   {Rear front} (Mil.), the rear rank of a body of troops when
      faced about and standing in that position.

   {Rear guard} (Mil.), the division of an army that marches in
      the rear of the main body to protect it; -- used also
      figuratively.

   {Rear line} (Mil.), the line in the rear of an army.

   {Rear rank} (Mil.), the rank or line of a body of troops
      which is in the rear, or last in order.

   {Rear sight} (Firearms), the sight nearest the breech.

   {To bring up the rear}, to come last or behind.

Source : WordNet®

rear admiral
     n : an admiral junior to a vice admiral