A stopper or cork is a cylindrical or conical closure used to seal a container, such as a bottle, tube or barrel. Unlike a lid or bottle cap, which encloses a container from the outside without displacing the inner volume, a bung is partially or wholly inserted inside the container to act as a seal.

A bung can be defined as "a plug or closure used to close an opening in a drum or barrel. It is called a plug when referring to a steel drum closure."

A glass stopper is often called a "ground glass joint" (or "joint taper"), and a cork stopper is called simply a "cork". Stoppers used for wine bottles are referred to as "corks", even when made from another material.

A common every-day example of a stopper is the cork of a wine bottle. When used to seal the bungholes of barrels, the stopper is called a bung. Other bungs, particularly those used in chemical barrels, may be made of metal and be screwed into place via threading.