Willows, also called sallows and osiers, form the genus Salix, are around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier, and some broader-leaved species are referred to as sallow. Some willows are low-growing or creeping shrubs; for example, the dwarf willow rarely exceeds 6 cm in height, though it spreads widely across the ground.
The generic name Salix comes from Latin and was already used by the Romans for various types of willow. A theory is that the word is ultimately derived from a Celtic language, sal meaning 'near' and lis meaning 'water', alluding to their habitat.