In fluid dynamics, an eddy is the swirling of a fluid and the reverse current (water) created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing water on the downstream side of the object. Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle. This phenomenon is most visible behind large emergent rocks in swift-flowing rivers.

Another possible turbulence is the vortex. This notion is rather aplied to gases than to fluids. Here, no void is created, but only an area of lower pressure, but again, a backflow causes the gas to rotate.

Eddy is also a name, usually an abbreviation of Edward.