French chemist Antoine Lavosier's 1789 famous element table: listing what he considered to be the 33 known elements, including caloric.

In chemistry, the caloric was a supposed form of matter held responsible for the phenomena of heat and combustion. [1] The caloric was considered as a hypthetical elastic fluid-like substance, that when introduced into bodies was considered as the the repulsive cause that separates the particles of matter from each other. [2]

Etymology
The term "calorique" (caloric) was introduced in 1787 by French chemist Guyton de Morveau, working in coordination with French chemist Antoine Lavoisier, Savoyard chemist Claude Berthollet, and French chemist Antoine de Fourcroy in a paper presenting the suggestions on the reformulation of the chemical nomenclature. [3] Lavoisier, in 1777, had formerly used the names igneous fluid and matter of heat for the hypothetical fluid. [4] The postulate of the caloric particle came to be known as the "caloric theory".

Combustion
In a 1780 memoir titled “On Combustion in General”, Lavoisier presented his new theory of combustion, with five key points:

(a) In combustion, there is disengagement of the matter of fire (caloric) or of light.
(b) A body can burn only in pure air [oxygen gas].
(c) There is “destruction or decomposition of pure air” and the increase in weight of the body burnt is exactly equal to the weight of the air “destroyed or decomposed”.
(d) The body burnt changes into an acid by addition of the substance which increases its weight.
(e) Pure air is compound of the matter of heat (caloric) or of light with a base; where in combustion, the burning body removes the base, which it attracts more strongly than does the matter of heat, and sets free the combined matter of heat, which appears as flame, heat, and light.

In this theory, as contrasted with the older phlogiston theory, which situated the matter of fire (phlogiston) in the combustible, the matter of heat was theorized to be located in pure air. [5]

References
1. Caloric (definition) - Merriam-Webster Collegiate Dictionary, 2000, version 2.5.
2. Lavoisier, Antoine. (1789). Elements of Chemistry, (pg. 5). London: G.G. and J.J. Robinsons.
3. Morveau, Guyton de. (1787). Méthode de Nomenclature Chimique (Method of Chemical Nomenclature), 31.
4. Collection of the French Academy of Sciences for that year, pg. 420.
5. Partington, J.R. (1957). A Short History of Chemistry, 3rd ed. (pg. 131-35). New York: Dover Reprint.