Stopper/ˈstɑːpər/ Definition 1 of 3Pro Tip 1/3Physical Object Refers to an actual object that physically obstructs a container or passage. Place the stopper in the sink to fill it with water. Pro Tip 2/3Variety of Shapes Stoppers can be made in various shapes to fit different containers. The flask came with a round glass stopper. Pro Tip 3/3Common Materials Stoppers are often made of rubber, glass, or cork. We need to replace the rubber stopper; it's worn out.