trenchant

As adjectives the difference between mordant and trenchant

is that mordant is having or showing a sharp or critical quality; biting; caustic; sarcastic; keen; severe while trenchant is fitted to trench or cut; gutting; sharp.

As a noun mordant

is any substance used to facilitate the fixing of a dye to a fibre; usually a metallic compound which reacts with the dye using chelation.

As a verb mordant

is to subject to the action of, or imbue with, a mordant.