The Collaborative International Dictionary of English v.0.48:

Monosaccharide \Mon`o*sac"cha*ride\, n. Also -rid \-rid\ .
   [Mono- + saccharide.] (Chem.)
   A simple sugar; any of a number of sugars (including the
   trioses, tetroses, pentoses, hexoses, etc.), not decomposable
   into simpler sugars by hydrolysis. Specif., as used by some,
   a hexose. The monosaccharides are all open-chain compounds
   containing hydroxyl groups and either an aldehyde group or a
   ketone group.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

monosaccharide
    n 1: a sugar (like sucrose or fructose) that does not hydrolyse
         to give other sugars; the simplest group of carbohydrates
         [syn: monosaccharide, monosaccharose, simple sugar]