Substances that ignite when they meet air are pyrophoric. They react with oxygen so rapidly that the heat released causes ignition without an external flame or spark, often as soon as they contact air—minutes or even seconds later. This is why such materials must be handled under inert atmospheres and stored in sealable containers to prevent exposure. The other options describe different ideas: a division classification is a regulatory category, not the behavior of catching fire on contact with air; spontaneously combustible materials will ignite in air due to self-heating or slow oxidation after exposure, not necessarily immediately on contact; oxidizers are substances that supply oxygen to support combustion rather than materials that ignite simply from air exposure.