U.S. Postage Stamps. - A postage stamp is adhesive paper evidence of a fee paid for postal services. Usually a small rectangle attached to an envelope, the stamp signifies the person sending it has fully or partly paid for delivery. Postage stamps are the most popular way of paying for retail mail; alternatives include prepaid-postage envelopes and postage meters. The study of postage stamps is philately. Stamp collecting is the hobby of collecting stamps.