The outermost covering of trees and many woody plants is called bark.
In everyday language, bark means the protective outside layer of a tree trunk or branch. Botanically, it refers broadly to all tissues outside the vascular cambium, the layer responsible for producing new wood and inner bark. Bark includes living inner tissues that transport sugars, as well as older outer tissues that help protect the plant.
As a stem ages, its outer protective covering commonly develops into a periderm. This includes cork, cork cambium, and phelloderm. The dead outer layers may collectively be called the rhytidome. Bark helps reduce water loss and shields living tissues from insects, disease, fire, physical damage, and temperature changes.
A common mix-up is using bark for every plant covering. Nonwoody plants usually have an epidermis rather than true bark, and roots can have different outer tissues. Bark is most accurately associated with trees, shrubs, and other woody stems and roots.