The caloric theory is an obsolete scientific theory that heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids. The "caloric theory" was superseded by the mid-19th century in favor of the mechanical theory of heat but nevertheless persisted in scientific literature until the end of the 19th century.
In the history of thermodynamics

History of thermodynamics

The history of thermodynamics is a fundamental strand in the history of physics, the history of chemistry, and the history of science in general...

, the initial explanations of heat were thoroughly confused with explanations of combustion

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

.