A vacuum breaker is an attachment that stops water from being sucked backward into the public drinking water system and is frequently seen on bibcock valves, toilet or urinal flush valves. This prevents contamination if the pressure in the public drinking water supply drops.
A vacuum breaker is made up of a plastic disc that is pushed forward by water pressure and closes small vent holes. If the supply pressure drops, the disc springs back, opening the vent holes (allowing outside air in) and preventing water backflow.
A more complex valve that accomplishes much the same purpose is the backflow preventer.
Vacuum relief valves are sometimes known as vacuum breakers.