Which of the following describes an air break?

An air break is defined as a discharge point that is positioned at a height above the flood rim of the receiving fixture. This configuration allows wastewater to flow from one vessel or plumbing system into another while simultaneously preventing potential backflow into the first vessel. By maintaining this separation, it protects the potable water supply from contamination. The flood rim is the highest point of the draining fixture, and positioning the discharge point above it means that should a backpressure occur, there is no direct pathway for the wastewater to siphon back, effectively creating a barrier to contamination.

The other options do not accurately describe the function or definition of an air break. While an air break does serve as a part of a system that helps prevent backflow, it is not a device in itself, and its primary characteristic is that it involves the physical height differential to establish that separation. Additionally, while an air break might relate to indirect waste applications, it’s more accurate to specify its role in the context of the flood rim. The concept of "unused space between two vessels" does not accurately represent the functional aspect of an air break. Therefore, the definition hinges on the crucial aspect of the discharge elevation relative to the flood rim, making it essential for backflow prevention in plumbing systems.