Alternating current, which is the flow of electrical current through a conductor, first in one direction, then in reverse. It is used exclusively in residential and commercial wiring because it provides greater flexibility in voltage selection and simplicity of equipment design.

The outside fan unit of the air-conditioning system that removes the heat from the Freon® gas and turns the gas back into a liquid, then pumps the liquid back to the coil in the furnace.

A wall that is mostly above grade and encloses conditioned space.

Acronym for acrylonitrile butadiene styrene; rigid black plastic pipe used only for drain lines.