Tanacetum is a genus of about 160 species of flowering plants in the aster family, Asteraceae, native to many regions of the Northern Hemisphere. They are known commonly as tansies. The name tansy can refer specifically to Tanacetum vulgare, which may be called the common tansy or garden tansy for clarity. The generic name Tanacetum means 'immortality' in Botanical Latin, since tansy was once placed between the burial sheets of the dead to repel vermin.

The family Asteraceae, with the original name Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae were first described in the year 1740. The number of species in Asteraceae is rivaled only by the Orchidaceae, and which is the larger family is unclear as the quantity of extant species in each family is unknown.