An eddy is a circular current of fluid, such as water or air, that flows contrary to the main flow direction. It is a swirling motion created when fluid moves around an obstacle or in turbulent flow, often producing a reverse current or whirlpool-like effect. Eddies are common in rivers behind rocks, in the ocean as rotating water masses, and even in the atmosphere as circular movements deviating from the general flow. They can range in size from small whirlpools to large oceanic features and play important roles in mixing fluids and transporting energy, nutrients, and other properties within the fluid.