Turbulence, in air travel, is a period of rough air patches which a plane travels through. Simply put, it is the unsteady or violent movement of air; in air travel, it may be the unsteady or violent movement of the plane through the air.
Though almost always not dangerous, it is just about the top fear for many travelers.
Turbulence has been categorized into some more specific terms: Wake turbulence is a form that follows the plane, as the jetwash causes such in the airplane’s wake. A more dangerous and less predictable form is clear-air turbulence.