The term 'breathable gas' can be summarized by the crossword solution AIR, which is a three-letter word. AIR is a mixture of gases that make up the Earth's atmosphere, and it is composed of nitrogen, oxygen, and other trace gases. Oxygen is the most important component of AIR for humans, as it is essential for respiration. Therefore, AIR is considered a breathable gas, as it contains enough oxygen for us to breathe.