1.NounThe eddying water under a slow-moving ship's counter, or a similar area of stationary fluid or gas in advance of a concave angle, which can occur when there is strong vertical density stratification (due to salinity or temperature or both), and can cause ships to become hard to control.
2.NounA region in a body of moving water (or similar body of fluid in motion) where the water (fluid) is relatively stationary, or stagnant.