The Collaborative International Dictionary of English v.0.48:

omelet \om"e*let\, omelette \om"e*lette\, n. [F. omelette, OF.
   amelette, alumete, alumelle, perh. fr. L. lamella. Cf.
   Lamella.] (Cookery)
   A dish consisting of eggs beaten up with a little flour,
   etc., and cooked in a frying pan until just set into a
   semisolid consistency; it may be folded around e.g. ham or
   cheese or jelly; as, a plain omelet. When additional
   ingredients are mixed in, the names of the ingredients may be
   mentioned in the name of the omelet; as, a ham and cheese
   omelet; a bacon and cheese omelet
   [1913 Webster + WordNet 1.5 + PJC]

   western omelet an omelet containing chopped pieces of ham,
      onion, and green peppers.
      [PJC]

WordNet (r) 3.0 (2006):

omelette
    n 1: beaten eggs or an egg mixture cooked until just set; may be
         folded around e.g. ham or cheese or jelly [syn: omelet,
         omelette]