1. A hypothetical substance formerly believed to be present in all combustible materials and released during burning; now discredited by modern chemistry.
Usage: archaic; historical; chemistry

Examples

• In the 18th century, scientists thought phlogiston was the invisible element that made things burn.
• The phlogiston theory was eventually replaced by the modern understanding of oxidation.
• Chemists once believed that metals contained phlogiston, which escaped when they were heated.
• The discovery of oxygen disproved the phlogiston hypothesis.
• Phlogiston is now studied in history of science courses as an example of a scientific theory that was later abandoned.