A forest is a large area dominated by trees. Hundreds of more precise definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing and ecological function.

The word forest comes from Middle English, from Old French forest (also forès) “forest, vast expanse covered by trees”; first introduced in English as the word for wild land set aside for hunting without the necessity in definition for the existence of trees.

The first known forests on Earth arose in the Late Devonian (approximately 380 million years ago), with the evolution of Archaeopteris. Archaeopteris was a plant that was both tree-like and fern-like, growing to 10 metres (33 ft) in height.

Forest ecology is the scientific study of the interrelated patterns, processes, flora, fauna and ecosystems in forests. The management of forests is known as forestry, silviculture, and forest management.

Forests provide a diversity of ecosystem services including, converting carbon dioxide into oxygen and biomass. A full-grown tree produces about 100 kg of net oxygen per year.