A mytical substance given off by fire during burning. Said to be the cause of rust. Theoretically a flame stops burning in an enclosed area because the air has become "phlogiston saturated".
Primarily a concept held during the middle ages, and associated with alchemaical "research".
A widely-researched phenomenon in the Middle Ages, phlogiston was thought to be an inherent substance in all things that could burn. When something was burned, phlogiston was visible in the form of flame, and was released into the air. In turn, people thought something would not burn without air because the air was already "full of phlogiston" and no more could be released into the air. In short, phlogiston was believed to be the substance that fire was made of.
These days, under influence of modern science, the concept of phlogiston is usually regarded as superstition.