A vacuum breaker is an attachment commonly placed on a bibcock valve or toilet or urinal flush valve, that prevents water from being siphoned backward into the public drinking water system. This prevents contamination should the public drinking water system's pressure drop.
A vacuum breaker typically contains a plastic disc that is pressed forward by water supply pressure, and covers small vent holes. Should the supply pressure drop, the disc springs back opening the vent holes (which let in outside air), and preventing backflow of water.
A more complex valve that accomplishes much the same purpose is the backflow preventer.
Vacuum relief valves are sometimes known as vacuum breakers.