ultraviolet protection factor: the effectiveness of clothing fabrics in protecting the skin from the sun’s ultraviolet UVA and UVB radiation, rated on a scale of increasing protectiveness, the number being the ratio of the radiation dose needed to produce minimal redness when wearing the fabric compared to the dose needed to produce the same redness when not wearing it: A shirt with a UPF of 50 will reduce your ultraviolet exposure by 50 times (98% UV blockage).
Compare SPF.

Origin of UPF

First recorded in 1985–90
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020