What is the definition of "phlogiston" in historical thermodynamics?

Phlogiston is defined historically as a hypothetical substance that was once believed to be released during the combustion process. This idea originated in the 17th century and was part of a widespread theoretical framework that attempted to explain fire and combustion phenomena. The phlogiston theory posited that materials containing phlogiston would release it when burned, leaving behind a residue that had lost its phlogiston content.

The concept of phlogiston was eventually replaced by the modern understanding of combustion involving oxidation and chemical reactions. The recognition that combustion is a result of reactions with oxygen fundamentally changed the field of chemistry and thermodynamics.

Other commonly held definitions do not apply in the context of phlogiston. For instance, considering phlogiston as a form of kinetic energy or a measure of heat energy does not align with how this term was used historically. Additionally, while combustion does produce gases, the term phlogiston itself is not associated with any specific emitted gas, but rather with the hypothetical substance believed to exist within burning materials.