A hinge is a small glassine-like strip, slightly longer but narrower than a finger nail, that is used for mounting postage stamps on an album page.

After wetting the muscilage side and folded over, (hence the term hinge) one half of the hinge is affixed to the back of the stamp and the other half is affixed to the page the stamp is to be mounted on. The hinges are removable, they peel right off ... but they do leave some disturbance on the gum that is readily detectable. Mint stamps with original gum that have never been hinged have a premium value over similar stamps that have been hinged. Hinging (generally speaking) does not significantly affect the value of used or cancelled stamps.