The Collaborative International Dictionary of English v.0.48:

Pentose \Pen"tose\, n. [Penta- + -ose.] (Chem.)
   Any of a group of sugars of the formula C5H10O5, such as as
   arabinose or ribose; -- so called from the five carbon
   atoms in the molecule. They are not fermented by yeast.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

pentose
    n 1: any monosaccharide sugar containing five atoms of carbon
         per molecule