Definition of 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 
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