Phlogiston was a hypothetical substance that was once believed to be released during combustion. It was proposed by the German alchemist Johann Joachim Becher in the 17th century to explain the loss of mass that occurs when objects burn. Becher believed that all combustible materials contained phlogiston, and that when they burned, the phlogiston was released and the material was left behind as ash.

The phlogiston theory was widely accepted for over a century, but it was eventually disproven by the experiments of Antoine Lavoisier in the late 18th century. Lavoisier showed that combustion does not release phlogiston, but instead consumes oxygen from the air. This led to the development of the modern theory of combustion, which states that combustion is a chemical reaction between a fuel and oxygen.