What term describes a substance that can ignite spontaneously in air?

The term that accurately describes a substance that can ignite spontaneously in air is "pyrophoric." Pyrophoric materials are particularly hazardous because they can spontaneously ignite upon exposure to oxygen or air without the need for an external ignition source. This characteristic makes them unique and defining within the realm of fire safety and hazardous materials.

In contrast, flammable and combustible materials require an external ignition source to catch fire, although they may ignite easily when given that source. Ignitable refers more broadly to substances that can catch fire when exposed to a flame or heat source, which does not encompass the specific property of spontaneous ignition characteristic of pyrophoric materials. Thus, the choice of pyrophoric is the most precise answer for a substance that ignites spontaneously in air.