1. By analogy with a molecule, a “glob” of fluid within the fluid mass that has a certain structure and life history of its own, the activities of the bulk fluid being the net result of the motion of the eddies. The concept is applied with varying results to phenomena ranging from the momentary spasms of the wind to storms and anticyclones. 2. Any circulation drawing its energy from a flow of much larger scale, and brought about by pressure irregularities, as in the lee of a solid obstacle. 3. In studies of the general circulation, departures of a field (e.g., temperature or relative vorticity) from the zonal mean of that field. 4. A closed circulation system produced as an offshoot from an ocean current. Eddies are the result of the turbulence of the oceanic circulation and are common throughout the World Ocean. The corresponding features in the atmosphere are the wind currents around high and low pressure disturbances. Oceanic cyclonic eddies have a shallow thermocline at the center and are therefore also known as cold-core eddies; anticyclonic eddies are associated with a depressed thermocline in the center and are also known as warm-core eddies. The most prominent eddies are those shed by western boundary currents, also known as rings; they are about 200 km in diameter and reach beyond a depth of 1500 m. Another class of eddies is produced by shear between currents flowing in opposing directions. These eddies tend to be smaller (10–50 km in diameter) and shallower.