An inhalation is an inward breath. When you swim to the surface of a pool after being under water, you'll take a deep inhalation.
The inhalation of gas fumes is bad for you, while inhalation of scents like peppermint can be very relaxing. If you've ever taken a yoga class, you know there's a lot of talk about breathing, inhalation and exhalation (breathing out). Inhalation comes from the verb inhale, "to breathe in," from the Latin inhalare, "breathe upon."