Reducing Sugar
A sugar with a free aldehyde or ketone group that can reduce mild oxidising agents.
Unit
Dimensionless
In depth
All monosaccharides and most disaccharides are reducing, because the ring can open to expose a carbonyl group. Sucrose is the notable exception, since both anomeric carbons are tied up in its glycosidic bond, so it must be hydrolysed before it will test positive.
Examples in the real world
Glucose giving a brick-red precipitate with Benedict's solution; sucrose giving none until hydrolysed.