1The seed of a mustard plant.
2Used allusively with reference to the mustard seed described in the New Testament (Matthew 13:31); (hence) a small thing capable of vast development. Especially in "grain of mustard seed".
The mustard seed (κόκκος σινάπεως) of the New Testament, spoken of as producing a tree, is probably the seed of the black mustard (Brassica nigra) which in Israel grows to a great height. However, some commentators have identified it with the toothbrush tree, Salvadora persica.
3US With singular agreement: very fine shot used for shooting birds while causing little injury to the plumage.
Late Middle English. From mustard + seed.