WordNet (r) 3.0 (2006):

hamburger
    n 1: a sandwich consisting of a fried cake of minced beef served
         on a bun, often with other ingredients [syn: hamburger,
         beefburger, burger]
    2: beef that has been ground [syn: ground beef, hamburger]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "hamburger":
   BLT, Dagwood sandwich, London broil, Salisbury steak, Sloppy Joe,
   Swiss cheese sandwich, T-bone steak, bacon-lettuce-tomato sandwich,
   beef, beef extract, beef tea, beefsteak, bifteck, bouillon, bully,
   bully beef, canape, charqui, cheeseburger, chipped beef,
   chopped steak, club steak, corned beef sandwich, cubed steak,
   double-decker sandwich, dried beef, egg salad sandwich,
   fillet steak, flank steak, grilled cheese sandwich, grinder,
   ground beef, ground chuck, ground round, ham sandwich, hamburg,
   hamburg steak, hoagy, hot dog, jerky, pastrami, pastrami sandwich,
   poor boy, porterhouse steak, rib steak, roast beef, round steak,
   rump steak, salt beef, sandwich, shell steak, sirloin steak, steak,
   submarine sandwich, suet, tenderloin steak, tuna salad sandwich,
   tunaburger