Acid free
From The Glossary of Digital Photography
Refers to paper or matting that is neutral (pH=7) or basic (pH>7) and has no acid. This is important because of the damaging effects of acid to paper, causing it to yellow and become brittle over time. Paper made from wood pulp has base material added to neutralize the natural acids found in wood. Buffering may be added to deal with any future acidity; this can be added during later processing. Acid free paper is more expensive and is generally used for higher-end products, such as fine art papers for photographs and in books, where greater longevity is desired.
