silphium

1 plural silphia \-ēə\ :  an extinct umbelliferous plant of the genus Ferula not definitely identifiable as to species but well known to the ancient Greeks and used by them medicinally

2 capitalized [New Latin, from Latin] :  a large genus of tall North American perennial herbs (family Compositae) having coarse heads of yellow flowers with fertile rays and broad flat winged achenes — see cup plant