The contents of a fire extinguisher (i.e. water, foam, powder, CO2, etc.) which extinguishes a fire when discharged upon it.
A fire extinguisher is a device designed to extinguish (put out) fires. There are different types of extinguishant designed to combat different types of fire.
Fire extinguishers are thoroughly tested on the classes of fire that the extinguishant inside is suitable for. The ratings given to the extinguisher indicate the maximum size of each type of fire that the extinguisher successfully tackled under test conditions.