SPF (sun protection factor): A number on a scale for rating the degree of protection provided by sunscreens. SPF stands for Sun Protection Factor.
SPF numbers on a package can range from as low as 2 to as high as 100. These numbers refer to the product's ability to screen or block out the sun's burning rays. It is a common mistake to assume that the duration of effectiveness of a sunscreen can be calculated simply by multiplying the SPF by the length of time it takes for him or her to suffer a burn without sunscreen, because the amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun. The amount of sun exposure depends upon a number of factors including the length of exposure, time of day, geographic location, and weather conditions.