Source : Webster's Revised Unabridged Dictionary (1913)

Marsupium \Mar*su"pi*um\, n.; pl. {Marsupia}. [L., a pouch],
   (Anat. & Zo["o]l.)
   (a) The pouch, formed by a fold of the skin of the abdomen,
       in which marsupials carry their young; also, a pouch for
       similar use in other animals, as certain Crustacea.
   (b) The pecten in the eye of birds and reptiles. See
       {Pecten}.

Source : WordNet®

marsupium
     n : an external abdominal pouch in most marsupials where newborn
         offspring are suckled
     [also: {marsupia} (pl)]