A plant of the genus sinapis, a pod-bearing, shrub-like plant, growing wild, and also cultivated in gardens. The little round seeds were an emblem of any small insignificant object. It is not mentioned in the Old Testament; and in each of the three instances of its occurrence in the New Testament (Matt. 13:31, 32; Mark 4:31, 32; Luke 13:18, 19) it is spoken of only with reference to the smallness of its seed. The common mustard of Palestine is the Sinapis nigra. This garden herb sometimes grows to a considerable height, so as to be spoken of as “a tree” as compared with garden herbs.
Mustard
2 verses
Matthew
Matthew 13:31
Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:
Matthew 13:32
Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
From Easton's Bible Dictionary
Matthew George Easton, 1897