The term UNBURNABLE is used to describe something that cannot be burned or destroyed by fire. This can be further defined by the crossword solutions NONFLAMMABLE (12 letters), INCOMBUSTIBLE (13 letters), and FIREPROOF (9 letters). NONFLAMMABLE materials are those that will not burn or ignite when exposed to a flame or spark. INCOMBUSTIBLE materials are those that will not burn under any circumstances, while FIREPROOF materials are those that are highly resistant to fire and heat.