The Chimney Safety Institute of America defines this word as, “Chimney and stovepipe deposits originating as condensed wood smoke, including tars, oils.”
Webster’s Dictionary defines it as, “A dark brown or black flammable tar deposited from especially wood smoke on the walls of a chimney.”
In my experience, I find most people do not realize that creosote is the by-product of anything that is burned in a fireplace or wood-burning stove or insert. This material is flammable and is the major reason to have a chimney cleaned.
In most cases, it is impossible to remove all the creosote in a chimney. Creosote can be stone hard and burns into the surfaces of a flue. But creosote is amazing because it oxidizes and changes chemical composition so that hard, burned on material will become cleanable in some amount of time.