The word "arboreal" means having to do with trees in some way. Arboreal plants are those that live in trees instead of on the ground. Such plants have made adaptations in order to survive in their lofty environments.
Some arboreal plants have become parasites, extracting all of their needs from the host plant. One example of such a plant is mistletoe, an evergreen plant that invades trees in many areas of the United States.
Epiphytes are members of the bromeliad family. These plants, also called "air plants," live on trees and other plants. They have evolved to be able to obtain all of their water and nutrients directly from the air.
The natural habitat for orchids is trees. They cannot be planted in soil, as their roots have adapted to need constant exposure to air, and they will die if the air flow is cut off.