#4615 σίναπι sinapi {sin'-ap-ee}

 perhaps from sinomai (to hurt, i.e. sting); TDNT - 7:287,1027; n n

—Greek Word Study (Transliteration-Pronunciation Etymology & Grammar)

 1) mustard, the name of a plant which in oriental countries grows
    from a very small seed and attains to the height of a tree, 10
    feet (3 m) and more; hence a very small quantity of a thing is
    likened to a mustard seed, and also a thing which grows to a
    remarkable size

—Thayer's (New Testament Greek-English Lexicon)

—Strong's (Greek Dictionary of the New Testament)

—NAS Exhaustive Concordance of the Bible with Hebrew-Aramaic and Greek Dictionaries

 AV - mustard seed 5; 5

—Exhaustive Concordance (KJV Translation Frequency & Location)