Medical Definition of mustard oil
1: a colorless to pale yellow pungent irritating essential oil that is obtained by distillation from the seeds usually of black mustard after expression of the fatty oil and maceration with water, that consists largely of allyl isothiocyanate, and that is used especially in liniments and medicinal plasters—compare sinigrin
2a: allyl isothiocyanateb: an isothiocyanate ester
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