A hose bibb is the outdoor faucet on the side of a house that a garden hose threads onto, also called a sillcock or wall hydrant. Plumbing code requires each one to carry a vacuum breaker, which stops contaminated water from being siphoned back into the home's drinking water.
A hose bibb is the outdoor faucet mounted on the exterior wall of a house, the one a garden hose screws onto. Plumbers and code books also call it a sillcock or a wall hydrant, but they all mean the same fixture: a valve that brings the home's water supply outside for a hose. The name comes from the threaded "bib" outlet that the hose connects to.
The hose bibb matters more than most people realize, because it is one of the most common spots in a home where dirty water can flow the wrong way. When a hose sits in a pool of fertilizer runoff, a bucket of soapy water, or a swimming pool, and the house water pressure drops for any reason, plain physics can pull that dirty water backward into the pipes. This is called backflow, and it can push contaminated water straight into the drinking supply. Picture a garden hose left in a wheelbarrow full of weed killer while a water main break drops the street pressure: without protection, that weed killer can get siphoned back toward the kitchen tap.
That risk is exactly why plumbing code treats a hose bibb as more than a simple faucet. Cross-connection rules require that every sillcock, hose bibb, or wall hydrant be protected by a vacuum breaker, a small device that lets air in to break the suction the moment water tries to reverse direction. Many modern hose bibbs come with the vacuum breaker built into the outlet. Older ones can be fixed with a threaded add-on vacuum breaker that screws onto the spout. The device has to meet a listed standard, and code does not treat it as optional, since it is the one part standing between a garden hose and the home's drinking water.
In cold climates, hose bibbs come in a frost-free design, where the actual shutoff valve sits back inside the heated wall rather than out at the exposed spout, so the water in the pipe cannot freeze and burst it. In the Phoenix area, hard freezes are rare, so a standard hose bibb usually works fine. Even here, though, the vacuum breaker requirement still applies, because the backflow risk it guards against has nothing to do with temperature.