The Collaborative International Dictionary of English v.0.48:

Omentum \O*men"tum\, n.; pl. Omenta. [L.] (Anat.)
   A free fold of the peritoneum, or one serving to connect
   viscera, support blood vessels, etc.; an epiploon.
   [1913 Webster]

   Note: The great, or gastrocolic, omentum forms, in most
         mammals, a great sac, which is attached to the stomach
         and transverse colon, is loaded with fat, and covers
         more or less of the intestines; the caul. The lesser,
         or gastrohepatic, omentum connects the stomach and
         liver and contains the hepatic vessels. The
         gastrosplenic omentum, or ligament, connects the
         stomach and spleen.
         [1913 Webster]

WordNet (r) 3.0 (2006):

omentum
    n 1: a fold of peritoneum supporting the viscera