white mustard (Sinapis alba)
Refers to the plant species Sinapis alba, used for its seeds and as a condiment.
White mustard seeds are used to make mustard.
Kanji spelling exists but is rarely used; usually written in kana.
Compound of 白 (shiro, 'white') + 芥子 (garashi, 'mustard'). The exact historical derivation is uncertain, but the name describes the pale seeds of the plant.