Pasteurized and dried egg whites (meringue powder also contains sugar and additives), powdered egg white is convenient--it requires no refrigeration and has a long shelf life--and it eliminates the risk of salmonella, especially useful in uncooked preparations like royal icing.
2 Tbs. reconstituted powdered egg whites = 3 large fresh egg whites
You can use fresh or pastuerized egg whites instead.
Look for powdered egg whites in the baking section of the supermarket.
For most cooking the powdered egg whites must be reydrated, usually in warm water; follow package of recipes directions.
Store in a cool, dry, place.