oxalic acid noun1 •A colourless, crystalline dicarboxylic acid, (COOH)₂, found in rhubarb, spinach and other plants, often as crystals of the calcium salt.If not sufficiently purified, a little hydrochloric or oxalic acid, or caustic potash may be put in the water, according as the stains are from grease or from ink. — 1880
