A reducing agent or reductant is a substance that either removes oxygen OR adds hydrogen in a given process. The term usually refers to the reducing of a complex substance into a simpler one. ie. ionic silver being reduced into metallic silver.
NB. heating the solution while making Colloidal Silver creates a reduction due to the absence of free oxygen in the solution. You can test this by taking room temp. Colloidal Silver (ionic) and Colloidal Silver created with heat and placing both in sunlight or exposing them to sodium chloride(salt).