Toluidine \To*lu"i*dine\, n. (Chem.) Any one of three metameric amido derivatives of toluene analogous to aniline, and called respectively {orthtoluidine}, {metatoluidine}, and {paratoluidine}; especially, the commonest one, or {paratoluidine}, which is obtained as a white crystalline substance. Note: It is used in the aniline dye industry, and constitutes the essential nucleus or radical of those dyes.