Phlogiston was a theoretical substance once believed to be released during the combustion of materials. Proposed in the 17th century, it was thought to be a fire-like element contained within combustible objects. When these materials burned, they were said to release phlogiston into the air, leaving behind a residue. The concept of phlogiston was widely accepted until the late 18th century when Antoine Lavoisier conducted experiments that led to the discovery of oxygen. Lavoisier's work ultimately disproved the phlogiston theory, establishing the modern understanding of combustion and chemical reactions, which rely on the role of oxygen rather than phlogiston.