The Collaborative International Dictionary of English v.0.48:

Tapioca \Tap`i*o"ca\, n. [Braz. tapioka: cf. Pg., Sp. & F.
   tapioca.]
   A coarsely granular substance obtained by heating, and thus
   partly changing, the moistened starch obtained from the roots
   of the cassava. It is much used in puddings and as a
   thickening for soups. See Cassava.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tapioca
    n 1: granular preparation of cassava starch used to thicken
         especially puddings