A treehouse is a building constructed at least partly among the branches of a tree, and using that tree or multiple trees as its primary means of structural support. Because of its construction it is almost necessarily limited in size, not only because of the limited room in a tree but because of the impracticality of building a permanent construction on a support which can grow or die. A treehouse may be enclosed or not, but should include some form of roof; a simple platform in a tree is not enough to count as a treehouse.

The supporting tree or trees carry the building rather than merely standing beside it. A ground-supported building near a tree has a different structural basis.

The sides can be enclosed or open, but some form of roof is required. An uncovered platform among branches is therefore insufficient.