A daylily, day lily or ditch-lily is a flowering plant in the genus Hemerocallis /ˌhɛmɪroʊˈkælɪs/, a member of the family Asphodelaceae, subfamily Hemerocallidoideae, native to Asia. Despite the common name, it is not taxonomically classified in the lily genus. 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. The plants are perennial, bulbous plants, whose common name alludes to its flowers, which typically last about a day.

Hemerocallis are herbaceous clump-forming perennials growing from rhizomes, some produce spreading stolons. They have a fibrous or fibrous-tuberous root system with contractile roots. The tuberous roots are used to store nutrients and water. The arching leaves are produced from the base of the plant (basal) and lack petioles, they are strap-like, long, linear lanceolate leaves and grouped into opposite fans. The crown is the small portion between the leaves and the roots. The large showy flowers are produced on scapes. The slightly irregular shaped flowers are arranged in helicoid cymes, or produced solitarily. The scapes of some species and cultivars produce small leafy proliferations arising from the nodes or in bracts. The proliferations are clones that root when planted.

Typically Hemerocallis flowers have three similar petals and three sepals, collectively called tepals, and each have a midrib. The centermost part of the flower, called the throat, may be a different color than the more distal areas of the tepals. Each flower has six stamens joined to the perianth tube, each with a two-lobed anther. The unequal stamen filaments are curved upward with the linear-oblong anthers dorsifixed. The superior ovary is green, with three chambers and the stigma is 3-lobed or capitate. The fruit is a capsule (often erroneously called a pod since botanical pods are found in Fabaceae). The fruits may have no seeds (sterile), or many relatively large, shiny, black, roundish seeds. The flowers of most species open in early morning and wither during the following night, possibly replaced by another one on the same scape the next day. Some species are night-blooming. The haploid number of chromosomes is eleven.

Despite their common name, daylilies are not true lilies (plants from the genus Lilium, family Liliaceae). Although the flowers of Hemerocallis and Lilium species have a similar shape, their growth habits, stems and leaf shapes are distinctive. Before 2009, the scientific classification of daylilies put them into the family Liliaceae. In 2009, under the APG III system, daylilies were removed from the family Liliaceae and assigned to the family Xanthorrhoeaceae, subfamily Hemerocallidoideae. Xanthorrhoeaceae was renamed in 2017 to Asphodelaceae in the APG IV system

As of January 2020^[update], Plants of the World Online recognized 16 species:

Two hybrids are recognized:

A number of hybrid names appear in the horticultural literature but are not recognized as valid by the World Checklist of Selected Plant Families. These include:

The name Hemerocallis comes from the Greek words ἡμέρα (hēmera) "day" and καλός (kalos) "beautiful".