The Collaborative International Dictionary of English v.0.48:

Sago \Sa"go\ (s[=a]"g[-o]), n. [Malay. s[=a]gu.]
   A dry granulated starch imported from the East Indies, much
   used for making puddings and as an article of diet for the
   sick; also, as starch, for stiffening textile fabrics. It is
   prepared from the stems of several East Indian and Malayan
   palm trees, but chiefly from the Metroxylon Sagu; also from
   several cycadaceous plants (Cycas revoluta, Zamia
   integrifolia, etc.).
   [1913 Webster]

   Portland sago, a kind of sago prepared from the corms of
      the cuckoopint (Arum maculatum).

   Sago palm. (Bot.)
   (a) A palm tree which yields sago.
   (b) A species of Cycas (Cycas revoluta).

   Sago spleen (Med.), a morbid condition of the spleen,
      produced by amyloid degeneration of the organ, in which a
      cross section shows scattered gray translucent bodies
      looking like grains of sago.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sago
    n 1: powdery starch from certain sago palms; used in Asia as a
         food thickener and textile stiffener