pyrethrum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A Eurasian perennial plant (Tanacetum coccineum) in the composite family, cultivated for its colorful flower heads and formerly used as a source of an insecticide.
• noun A closely related perennial plant (Tanacetum cinerariifolium), commercially cultivated as the source of an insecticide.
• noun An insecticide made from the dried flower heads of any of these plants.

from The Century Dictionary.

• noun A plant of the genus Pyrethrum; feverfew.
• noun [capitalized] A former genus of composite plants of the tribe Anthemideæ, now included as part of the section Pyrethra in the genus Chrysanthemum, from which it was distinguished by achenes nearly equally from five- to ten-ribbed and crowned with a pappus, characters now known to vary in the same species.
• noun A powdered preparation of pyrethrum, used as an insectifuge. Also called pyrethrum-powder. See insect-powder and buhach.
• noun In pharmacy, the Anacyclus Pyrethrum, or pellitory-of-Spain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several perennial African plants of the genus Chrysanthemum, especially Chrysanthemum cinerariaefolium
• noun organic chemistry Any of several insecticides obtained from these plants; pyrethrin

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun made of dried flower heads of pyrethrum plants
• noun white-flowered pyrethrum of Balkan area whose pinnate leaves are white and silky-hairy below; source of an insecticide; sometimes placed in genus Chrysanthemum
• noun used in former classifications for plants later placed in genus Chrysanthemum and now often included in genus Tanacetum
• noun spring-flowering garden perennial of Asiatic origin having finely divided aromatic leaves and white to pink-purple flowers; source of an insecticide; sometimes placed in genus Chrysanthemum