What is "mustard seed"

n. black or white seeds ground to make mustard pastes or powders

Mustard seeds are the small round seeds of various mustard plants. The seeds are usually about in diameter and may be colored from yellowish white to black. They are important herbs in many regional foods and may come from one of three different plants: black mustard ( Brassica nigra), brown Indian mustard ( B. juncea), or white mustard ( B. hirta/Sinapis alba).

An archaic name for the seed is eye of newt. Often misunderstood for an actual eye of a newt, this name has been popularly associated with witchcraft ever since it was mentioned as an ingredient to a witch's brew in Shakespeare's famous play MacBeth.