The definition of a flue is a duct, pipe, or opening in a chimney for conveying smoke, gas, and air to the outside atmosphere. A flue is typically made of metal, ceramic, or other heat-resistant material and is usually lined with a heat-resistant material such as refractory brick or ceramic tile. The flue is an important part of a chimney system, as it allows the smoke, gas, and air to be safely vented from the home. Without a flue, the smoke, gas, and air would remain in the home, creating a dangerous and unhealthy environment. The flue also helps to keep the chimney from becoming blocked, which can cause dangerous chimney fires. The flue is also important for maintaining the proper draft in the chimney, which helps to ensure that the fire burns efficiently and safely.