A daylily, day lily or ditch-lily is a flowering plant in the genus Hemerocallis, a member of the family Asphodelaceae, subfamily Hemerocallidoideae, native to Asia. Despite the common name, it is not, in fact, a lily, nor does it specifically grow in ditches. Gardening enthusiasts and horticulturists have long bred Hemerocallis species for their attractive flowers; a select few species of the genus have edible petals, while some are extremely toxic. Thousands of cultivars have been registered by the American Daylily Society, the only internationally recognized registrant according to the International Code of Nomenclature for Cultivated Plants (ICNCP).. The plants are perennial, bulbous plants, whose common name alludes to its flowers, which typically last about a day.

Asphodelaceae is a family of flowering plants in the order Asparagales. Such a family has been recognized by most taxonomists, but the circumscription has varied widely. In its current circumscription in the APG IV system, it includes about 40 genera and 900 known species. The type genus is Asphodelus.