A fire retardant is a substance other than water that reduces flammability

Flammability

Flammability is defined as how easily something will burn or ignite, causing fire or combustion. The degree of difficulty required to cause the combustion of a substance is quantified through fire testing. Internationally, a variety of test protocols exist to quantify flammability...

 of fuels or delays their 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...

. This typically refers to chemical retardants but may also include substances that work by physical action, such as cooling the fuels; examples of these include fire-fighting foams and fire-retardant gel

Fire-retardant gel

Fire-retardant gels are superabsorbent polymer slurries with a "consistency almost like petroleum jelly." Used as fire retardants, they can be used for structure protection and in direct-attack applications against wildfires.- History :...

s. The name fire retardant may also be applied to substances used to coat an object, such as a spray retardant to prevent Christmas trees from burning.