Reduction: Of Substrate By Reagent
A substrate is deemed to be reduced by a reducing agent if the reducing agent (1) adds electrons to the substrate by single electron transfer or SET, (2) adds electron rich hydrogen (as H2, or H-) to the substrate, (3) adds electron density from the substrate by reversing polarisation by removing electronegative oxygen or (4) electronegative halogen atoms.

For more information look in the Chemogenesis webbook section on redox chemistry.