Fumigants are pesticides that:

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Fumigants work as vapors that move through the entire space to reach pests hidden in cracks, soil, or structure materials. Because they disperse as a gas, the area being treated must be tightly sealed to keep the gas in long enough to kill pests and to prevent dangerous leaks. Their high toxicity means strict safety controls, training, personal protective equipment, and gas monitoring are essential during use.

This is different from applying a liquid to leaves, which is a foliar spray acting on surface or systemic uptake rather than a gas that fills the space. It also differs from solids placed in soil, which are typically granules or pellets that act by contact or uptake rather than by vapor-phase movement. And fumigation is not limited to aerosols for indoor surfaces only; the defining feature is gas movement through the space to reach pests, whether in buildings, soils, or other enclosed areas.