What term describes materials that ignite spontaneously in air?

The term that describes materials capable of igniting spontaneously in air is "pyrophoric." Pyrophoric substances have the unique property of igniting when exposed to oxygen at room temperature, without the need for an external ignition source. This characteristic poses significant safety risks in handling and storage, as pyrophoric materials can combust quickly and unexpectedly upon contact with air.

In contrast, flammable materials require a certain ignition temperature or spark to catch fire, while combustible substances may ignite at higher temperatures but are not necessarily as reactive as pyrophoric ones. Explosive materials, on the other hand, can undergo rapid chemical reactions but are defined by their ability to create a shockwave or blast, which differs from the self-igniting capacity of pyrophoric materials. Hence, "pyrophoric" is the most accurate term for materials that spontaneously ignite in air.