A non-reducing sugar is one kind of carbohydrate that is not oxidised by a weak oxidising agent (an oxidising agent that oxidises aldehydes but not alcohols, such as the Tollen's reagent) in a basic aqueous solution.
C12H22O11
Sucrose(A)
(Non-reducingsugar)
+H2O⟶
C6H12O6
D−glucose
+
C6H12O6
D−fructose
While all others, i.e. glucose, fructose and galactose are reducing sugar