Coumarin \Cou"ma*rin\ (k[=oo]"m[.a]*r[i^]n), n. [F., fr. coumarou, a tree of Guiana.] (Chem.) The concrete essence of the tonka bean, the fruit of {Dipterix (formerly Coumarouna) odorata} and consisting essentially of coumarin proper, which is a white crystalline substance, {C9H6O2}, of vanilla-like odor, regarded as an anhydride of coumaric acid, and used in flavoring. Coumarin in also made artificially.