An air gap is defined as a separation between what two entities?

An air gap is specifically recognized as a physical separation between a potable water supply and an open vessel or any other source that could potentially contaminate the water. In this context, the term "air gap" serves a critical function in preventing backflow, which is the undesirable reversal of flow in a system.

When a potable water pipe is connected to an open vessel, such as a sink or a bowl, there must be a sufficient vertical distance maintained between the two to ensure that if the open vessel were to overflow or if the pressure in the potable system were to drop, contaminated water cannot siphon back into the clean water supply. This is crucial for maintaining the safety and hygiene of drinking water.

In contrast, the other options do not pertain to the essential definition and purpose of an air gap. Although a potable water pipe's relationship with a drinking glass or a shower might touch on similar safety concerns, they do not accurately depict the specific requirement of maintaining a vertical separation that characterizes an air gap. Hence, the correct answer reflects the fundamental principle of preventing contamination through a clear, defined separation with respect to potable water systems.