The Collaborative International Dictionary of English v.0.48:

Service cap \Serv"ice cap\ or Service hat \Serv"ice hat\ (Mil.)
   A cap or hat worn by officers or enlisted men when full-dress
   uniform, or dress uniform, is not worn.

   Note: In the United States army the service cap is round,
         about 31/2 inches high, flat-topped, with a visor. The
         service hat is of soft felt of khaki color, with broad
         brim and high crown, creased down the middle.
         [Webster 1913 Suppl.]