CH2=CHCH2NCS; volatile mustard CHallylisosulfocyanate; isothiocyanic allyl ester; obtained from Brassica nigra or produced synthetically; a vesicant, used in 10% solution in 50% alcohol as a counterirritant in neuralgia. Gives mustard its characteristics flavor and aroma. See Also: mustard oil. Syn: volatile mustard oil.