Motion sickness, also known as kinetosis, is an ailment characterized by nausea, dizziness, and fatigue due to the confusion or disagreement between the visual perception of movement and the sense of movement coming from the vestibular system. This condition is a usual problem among travelers, whether they travel by car, airplane, trains, and more often in boats. It also known through several other terms and kinds that determine the cause of motion sickness, such as carsickness, seasickness, space sickness, simulation sickness, or air sickness.

The brain recognizes movement by obtaining signals coming from the eyes, inner ears, joints, and muscles. If it gets conflicting or contradictory signals, motion sickness occurs. It starts abruptly with a queasy or uncomfortable sensation and cold sweat, then leads to dizziness, nausea, and vomiting.

A common explanation says that motion sickness emerged from the body's defense mechanism to combat neurotoxins. A part of the brain called the area postrema triggers vomiting when it detects poisons and is also in charge of clearing up conflicts on balance and vision. When the brain receives contradictory signals (such that movement is felt but not seen), it concludes that either of the two organs are hallucinating, which is probably due to the presence of a poison. Vomiting is then induced to clear the suspected toxic substance.

Motion sickness has a number of natural and chemical treatments, such as looking out the vehicle window during travel (to adjust the body's inner sense of balance by visually confirming motion), closing the eyes or sleeping when traveling by boat (to prevent the contradiction between what is seen and what is felt), and taking specific medications for motion sickness.