叶是高等植物的营养器官，侧边发育自植物的茎的叶原基。叶内含有叶绿体，是植物进行光合作用的主要场所。同时，植物的蒸散作用是通过叶的气孔实现的。

叶只出现在真正的茎上，即只有维管植物才有叶。蕨类、裸子植物和被子植物等所有高等植物都有叶。相对地，苔藓植物、藻类、真菌和地衣则没有叶。在这些扁平体（Thallus）中只能找到与叶相似的结构，但只能作为类似物（Analoga）。

但有人认为，上述的叶的外延，只是狭义的。广义的叶应该指所有能行光合作用的组织结构。但有一部分的茎为了不让水分被蒸散掉，而演变出如仙人掌般针状的叶子。

A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Foliage is a mass noun that refers to leaves collectively.

Typically a leaf is a thin, dorsiventrally flattened organ, borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the (palisade mesophyll), is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinctive upper (adaxial) and lower (abaxial) surfaces that differ in colour, hairiness, the number of stomata (pores that intake and output gases), epicuticular wax amount and structure and other features.