Ancient woods are areas of woodland that have persisted since 1600 in England, Wales and Northern Ireland, and 1750 in Scotland. This is when maps started to be reasonably accurate so we can tell that these areas have had tree cover for hundreds of years. They are relatively undisturbed by human development.

An old-growth forest [a] (also referred to as primary forest) is a forest that has developed over a long period of time without disturbance. Due to this, old-growth forests exhibit unique ecological features.