what is an air gap for a faucet
Air gap (plumbing) A simple example is the space between a wall mounted faucet and the sink rim (this space is the air gap). Water can easily flow from the faucet into the sink, but there is no way that water can flow from the sink into the faucet without modifying the system. This arrangement will prevent any contaminants in the sink from flowing into the potable water system by siphonage and is the least expensive form of backflow prevention.
Acetabulum The acetabulum /æsəˈtæbjʊləm/ (cotyloid cavity) is a concave surface of a pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.[1][2]