Motion sickness: A disorder of the sense of balance and equilibrium and, hence, the sense of spatial orientation that is caused by repeated motion such as from the swell of the sea, the movement of a car, or the motion of a plane in turbulent air. Motion sickness is due to irritation of a portion of the inner ear called the labyrinth. The symptoms of motion sickness include nausea, vomiting, and vertigo. Other common signs of motion sickness are sweating and a general feeling of discomfort and not feeling well (malaise). Symptoms usually stop when the motion that causes it ceases. However, some people suffer symptoms for even a few days after the trip is over.