sun protection factor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A measure of the degree to which a sunscreen protects the skin from ultraviolet radiation, expressed as a number representing the ratio of the minimal ultraviolet dose required to produce erythema with and without sunscreen.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun an indication of how long a sunscreen remains effective when applied to the skin

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the degree to which a sunscreen protects the skin from the direct rays of the sun