A stopper is an object used to plug or close an opening, preventing whatever's inside from leaking out or letting anything unwanted in. The cork in a bottle of olive oil is a stopper. The rubber plug in a bathtub drain is a stopper. Laboratory scientists use glass stoppers in chemical bottles to keep liquids fresh and prevent spills.

Stoppers work by fitting snugly into an opening, creating a seal. Some stoppers, like wine corks, need to be pulled or twisted out. Others, like sink stoppers, have chains attached so you can easily pull them up to let water drain. Before screw-on caps became common, most bottles used stoppers made from cork, rubber, or glass.

The word can also mean something or someone that stops an action. In soccer, a goalkeeper is sometimes called a stopper because they stop the other team from scoring. A show-stopper is a performance so amazing that the audience stops the show with their applause. When you hit a “work-stopper” on a project, you've run into a problem that stops your progress until you solve it.