Source : Webster's Revised Unabridged Dictionary (1913)

Pyrethrin \Pyr"eth*rin\, n. [NL. Pyrethrum, generic name of
   feverfew, Gr. ? feverfew.] (Chem.)
   A substance resembling, and isomeric with, ordinary camphor,
   and extracted from the essential oil of feverfew; -- called
   also {Pyrethrum camphor}.