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Motion sickness or kinetosis is a condition in which a disagreement exists between visually perceived movement and the vestibular system's sense of movement. Depending on the cause it can also be referred to as seasickness, carsickness, simulation sickness, airsickness, or space sickness.[1]

Dizziness, fatigue, and nausea are the most common symptoms of motion sickness.[2] Sopite syndrome is also a side effect of motion sickness. In fact, nausea in Greek means seasickness (naus means ship).[3][4] If the motion causing nausea is not resolved, the sufferer will frequently vomit. Unlike ordinary sickness, vomiting in motion sickness tends not to relieve the nausea.[1]

About 33% of people are susceptible to motion sickness even in mild circumstances such as being on a boat in calm water, although nearly 66% of people are susceptible in more severe conditions.[1][5] Approximately 50% of the astronauts in the U.S. space program have suffered from space sickness.[6] Individuals and animals without a functional vestibular system are immune to motion sickness.[7]

Motion sickness on the sea can result from being in the berth of a rolling boat without being able to see the horizon. Sudden jerky movements tend to be worse for provoking motion sickness than slower smooth ones, because they disrupt the fluid balance more.[1] A "corkscrewing" boat will upset more people than one that is gliding smoothly across the oncoming waves. Cars driving rapidly around winding roads or up and down a series of hills will upset more people than cars that are moving over smooth, straight roads. Looking down into one's lap to consult a map or attempting to read a book while a passenger in a car may also bring on motion sickness.

The most common hypothesis for the cause of motion sickness is that it functions as a defense mechanism against neurotoxins.[7] The area postrema in the brain is responsible for inducing vomiting when poisons are detected, and for resolving conflicts between vision and balance. When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. As a result of the disconcordance, the brain will come to the conclusion that one of them is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin.