If you are airsick, your stomach feels bad because the plane is moving — climbing, dropping, turning, shaking. It is the same family of feeling as being seasick on a boat or carsick in a car: your eyes say one thing, the balance parts in your ears say another, and your body answers with nausea, a cold sweat, sometimes vomiting. People say it in ordinary talk, not as a doctor's word: a child can say “I feel airsick.” You will also hear it used a bit loosely. Someone who feels awful on a plane from stale cabin air, from nerves, or from a heavy meal may still call it being airsick, though actually the tidy meaning is the movement, not those other plane miseries. It is an adjective: you are airsick, you feel airsick, you get airsick.