Non moving or very slightly moving water, often associated with adjacent moving water.
(SLACK TIDE): The state of a TIDAL CURRENT when its velocity is near zero, especially the moment when a reversing current changes its direction and its velocity is zero. The term is also applied to the entire period of low velocity near the time of turning of the current when it is too weak to be of any practical importance in navigation. The relation of the time of slack water to the tidal phases varies in different localities. In some places slack water occurs near the times of high and low water, while in other localities the slack water may occur midway between HIGH and LOW WATER.
The period at high or low tide when there is little or no current
The state of a tidal current when its speed is near zero, especially the moment when a reversing (rectilinear) current changes direction and its speed is zero. For a theoretical standing tidal wave, slack water occurs at the times of high water and low water, while for a theoretical progressive tidal wave, slack water occurs midway between high and low water.
the occurrence of relatively still water at the turn of the (low) tide
Water at the top or bottom of the tides. Usually associated with no currents.
Minimum velocity of a tidal current (sometimes abbreviated "slack"). Usually the optimum time to dive wrecks etc.
Slack water is the time during which no appreciable tidal current is flowing in a body of water.