Combustion is the essential fact which is at the basis of that assemblage of visible phenomenon which we call fire; combustion being the continuous ch more..Combustion is the essential fact which is at the basis of that assemblage of visible phenomenon which we call fire; combustion being the continuous chemical combination of a substance with some element, as oxygen, evolving heat, and extending from slow processes, such as those by which the heat of the human body is maintained, to the processes producing the most intense light also, as in a blast-furnace, or on the surface of the sun. Fire is always attended with light, as well as heat; blaze, flame, etc., designate the mingled light and heat of a fire. Combustion is the scientific, fire the popular term. A conflagration is an extensive fire. Compare LIGHT.(verb)