1. (loan) (noun) willow tree, crack willow, Salix fragilis - grows to a large, mainly single-trunked tree with an extensive root system. Leaves long and narrow, finely toothed, hairless, bright green above and paler below. Grey willow, pussy willow, Salix cinerea - shrub or small tree 2-10 m tall. Leaves shorter and wider towards the tip, pale green above and grey and finely hairy below with slightly wavy or toothed edges.