Mallee are trees or shrubs, mainly certain species of eucalypts, which grow with multiple stems springing from an underground lignotuber, usually to a height of no more than 10 m (33 ft). The term is widely used for trees with this growth habit across southern Australia, in the states of Western Australia, South Australia, New South Wales and Victoria, and has given rise to other uses of the term, including the ecosystems where such trees predominate, specific geographic areas within some of the states and as part of various species' names.

A lignotuber is a woody swelling of the root crown possessed by some plants as a protection against destruction of the plant stem, such as by fire. Other woody plants may develop basal burls as a similar survival strategy, often as a response to coppicing or other environmental stressors. However, lignotubers are specifically part of the normal course of development of the plants that possess them, and often develop early on in growth. The crown contains buds from which new stems may sprout, as well as stores of starch that can support a period of growth in the absence of photosynthesis. The term "lignotuber" was coined in 1924 by Australian botanist Leslie R. Kerr.