An eddy is a circular current of water.

They walked hand in hand to the bridge, leant over the parapet, and watched the river eddying below.

Far off is seen, above the billowy mere, / Trinacrian AEtna, and the distant roar / of ocean and the beaten rocks we hear, / and the loud burst of breakers on the shore; / high from the shallows leap the surges hoar, / and surf and sand mix eddying.

You may have seen an eddy if you've ever gone canoeing and you see a small whirlpool of water while you paddle through the water.