树是具有木质树干及树枝的植物，可存活多年。一般将乔木称为树，主干，植株一，分枝距离地面较高，可以形成树冠。树有很多种。

俗语中也有将比较大的灌木称为“树”的，如石榴树、茶树等。

树在减少土地侵蚀及调整气候上相当的重要，树可以从空气中吸收二氧化碳，将大量的碳保存在组织内。树木和森林是许多物种的栖息地。热带雨林是世界上生物多样性最丰富的地方之一。树可以提供遮阴及保护，木材可供建筑用，木炭可以用来加热及烹煮，果子可以用来作为食物。在世界各地的森林面积正在下降，目的是要增加可以农业使用的土地。由于树的长寿及实用，在许多神话中也有树的出现。

In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are usable as lumber or plants above a specified height. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants to compete for sunlight. In looser senses, the taller palms, the tree ferns, bananas and bamboos are also trees. Trees tend to be long-lived, some reaching several thousand years old. The tallest known tree, a coast redwood named Hyperion, stands 115.6 m (379 ft) high. Trees have been in existence for 370 million years. It is estimated that there are just over 3 trillion mature trees in the world.