An air gap is an open vertical space between a water outlet and the flood-level rim of the fixture below it, such as the gap between a faucet spout and the sink rim. That open space physically blocks dirty water from being siphoned back into the clean water supply, making it the simplest and strongest form of backflow protection.
An air gap is one of the oldest and most reliable ideas in plumbing: leave an open space, and water cannot flow backward across it. Formally, it is the unobstructed vertical distance between the outlet of a water supply, like a faucet spout, and the flood-level rim of the fixture it fills, like the edge of the sink. You already have air gaps in your home without thinking about them. The space between your kitchen faucet and the rim of the sink is an air gap.
The point of that gap is backflow prevention. Backflow is when used or contaminated water gets pulled backward into the clean water supply, usually because pressure in the supply line drops suddenly. If a faucet spout sat down inside dirty dishwater, a pressure drop could siphon that dirty water up into the pipes. The air gap makes that impossible. Water cannot climb up through open air, so the clean supply stays separated from whatever is in the basin. Because it is a physical break rather than a moving part that can wear out or stick, an air gap offers the highest level of protection of any backflow method.
The most familiar dedicated example is a dishwasher air gap, the small chrome cap you often see on the back of a kitchen sink. A dishwasher drains dirty water out through a hose. Without protection, a clog in the sink drain could push that dirty water back into the dishwasher and onto clean dishes. The air gap fitting routes the drain line up to counter height and across an open break, so sink water can never flow back into the dishwasher. For example, if the disposal below clogs and backs up, water spills out of the air gap cap onto the counter instead of flowing into the dishwasher.
Plumbing code requires an air gap in specific spots where a backflow risk exists, part of broader cross-connection control rules. The Uniform Plumbing Code that Phoenix uses calls for an air gap on new dishwasher installations, and air gaps are required on many other outlets that could otherwise submerge in contaminated water. The gap has to be tall enough to work, generally at least twice the diameter of the supply outlet, so a splash or a rising basin cannot bridge it.