Pyrethrum refers to several Old World plants of the genus Chrysanthemum

Chrysanthemum

Chrysanthemums, often called mums or chrysanths, are of the genus constituting approximately 30 species of perennial flowering plants in the family Asteraceae which is native to Asia and northeastern Europe.-Etymology:...

(e.g., C. coccineum) which are cultivated as ornamentals for their showy flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

 heads. Pyrethrum is also the name of a natural insecticide

Insecticide

An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

 made from the dried flower heads of C. cinerariifolium and C. coccineum.
Some members of the Chrysanthemum genus, such as the following two, are placed in the Tanacetum

Tanacetum

Tanacetum is a genus of about 160 species of flowering plant in the family Asteraceae, native to many regions of the Northern Hemisphere.Common names include Tansy , Costmary , and Feverfew ; several other species are also known as tansies.Tanacetum species...

genus instead by some botanists.