A duct is a tube or channel that is used to transport air, fluids, or other materials from one place to another. In buildings, ducts are commonly part of the heating, ventilation, and air conditioning (HVAC) systems, helping to distribute warm or cool air throughout different rooms. Ducts can be made from various materials, including metal, plastic, or fiberglass, and come in different shapes and sizes. Proper duct design and installation are essential for energy efficiency and maintaining good indoor air quality, as they help ensure that air flows smoothly and evenly throughout a space.