mustard tree n 1: evergreen South American shrub naturalized in United States; occasionally responsible for poisoning livestock [syn: {tree tobacco}, {Nicotiana glauca}] 2: glabrous or pubescent evergreen shrub or tree of the genus Salvadora; twigs are fibrous and in some parts of the world are bound together in clusters and used as a toothbrush; shoots are used as camel fodder; plant ash provides salt [syn: {toothbrush tree}, {Salvadora persica}]