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
Part of Speech: noun neuter
Citing in TDNT: 7:287, 1027
Perhaps from σίνομαι sinomai (to hurt, that is, sting); mustard (the plant): - mustard.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root σίναπι; click on the second column to search for that grammatical form of the root σίναπι; click on the third column to search for that word and grammatical form; click on the total to search for the root σίναπι.