Slack water (also called slack tide) is the time at which the tidal current (i.e. the tidal component of the current velocity) is smallest.
Slack water occurs twice during a tidal cycle. In coastal waters (flow influenced by friction and boundaries), one slack water occurs closer to low water (low-water slack tide, LWS) and another slack water occurs closer to high water (high-water slack tide, HWS). LWS marks the end of the ebb flow and the start of flood flow; HWS marks the end of the flood flow and the start of the ebb flow.
In uniform unidirectional flow, slack water corresponds to flow reversal, i.e. zero flow velocity. In non-uniform unidirectional flow, flow reversal near the seabed occurs before flow reversal near the water surface. In the open sea, the term 'slack water' is rarely used because the current rotates during the tidal cycle and is never equal to zero.
See also Definition of ebb and flood (tide).