noun. A ketone, C₁₀H₁₆O, isomeric with camphor, which it resembles in general chemical properties. The dextro-form occurs in fennel-oil, the levo-form in thuja-oil.

noun. A monoterpenoid ketone present in the essential oil of fennel

"Most fennel seed comes from sweet fennel varieties (p. 407) and tastes sweet; seed from the less cultivated types are also bitter due to the presence of a particular terpene (fenchone)."