U.L.    Underwriters Laboratories, a private research firm that attempts to classify and determine the safety of various electrical products.
Ultraviolet degradation    A reduction in certain performance limits caused by exposure to ultraviolet light.
Underlayment      A secondary roofing layer that is waterproof or water-resistant installed on the roof deck and beneath shingles or other roof-finishing layer.
UV rays     Ultraviolet rays from the sun.