Disequilibrium is the inability to maintain a sense of balance. In a standing person, it usually leads to dizziness and nausea.
Human beings primarily rely on their vision to maintain their balance, although they also rely on the workings of canals in the inner ear that sense motion and position, and on proprioception - the ability for the body to sense where all of its parts are without looking at them. Disequilibrium can be the result of several causes within this system, such as vision problems, infections or disorders of the inner ear, fatigue, the use of alcohol or drugs, numbness in the legs, or even sudden movement.
If the feeling persists or recurs without warning, it can be a symptom of an underlying disorder of the brain or ear.