The term 'stamp backing' refers to the adhesive material used to attach a stamp to an envelope or other surface. The two most common materials used for this purpose are GLUE and GUM. GLUE is a liquid adhesive that is applied to the back of the stamp and then allowed to dry, while GUM is a sticky substance that is applied directly to the surface of the envelope. Both materials provide a secure hold for the stamp and ensure that it will remain in place until it is removed.