A liana is any of various long-stemmed, woody vines that are rooted in the soil at ground level and use trees, as well as other means of vertical support, to climb up to the canopy to get access to well-lit areas of the forest. Lianas are especially characteristic of tropical moist deciduous forests and rainforests, including temperate rainforests. There are also temperate lianas, for example the members of the Clematis or Vitis (wild grape) genera. Lianas can form bridges amidst the forest canopy, providing arboreal animals with paths across the forest. These bridges can protect weaker trees from strong winds. Lianas compete with forest trees for sunlight, water and nutrients from the soil. Well-known lianas include monkey ladder (Entada gigas) and "water vine", which can refer either to Cissus hypoglauca or to members of the genus Doliocarpus.
The term "liana" is not a taxonomic grouping, but rather a description of the way the plant grows, and lianas may be found in many different plant families. One way of distinguishing lianas from trees and shrubs is based on the stiffness, specifically, the Young's modulus of various parts of the stem. Trees and shrubs have young twigs and smaller branches which are quite flexible and older growth such as trunks and large branches which are stiffer. A liana often has stiff young growths and older growth, at the base of the stem, which is more flexible.