What is the duct in an air conditioning system?

The "duct" in an air conditioning and/or heating system is the sheet metal conduit that carries the conditioned air to the perimeter of a structure and back to the HVAC unit. Typically in a residential application it appears as a square or rectangular metal trunk line centrally located on the ceiling of a basement, with round metal run outs coming off the top of the trunk line and running towards the perimeter of the house.