"Auto vent" or "In-line vent"

An auto vent (also called "in-line vent) is used to prevent accidental siphoning of the washing machine as it refills. This product allows air to enter the line, but does not allow water to exit. They can be used inside a building or out.  In standard plumbing situations they're used for venting an island sink, or in a place where running a vent pipe up to the roof is not feasible.