A breathing tube or breathing device, as the name suggests, is a structure which is designed to assist with respiration. These are often used by species who are living or working among other beings who have different environmental requirements for breathing mixtures.

Barzans have to wear breathing devices when they leave their homeworld Barzan II, the device provides them with the gases that are found in the atmosphere of their homeworld and that are toxic to other species. (TNG: "The Price")

The Benzenites are known to have breathing tubes connected to their heads; if the tubes were removed, the Benzenite individual would die. (DS9: "Apocalypse Rising")