Semidiurnal And Diurnal Tides
In most places, tides are semidiurnal (twice-daily), meaning that there are two tidal cycles (with one high tide and one low apiece) each day. In other words, during a typical day the tides reach their highest point along the shore and their lowest point twice each day. The high-water level reached during one of the high tide stages is usually higher than the other high point, and the low water level reached during one of the low tide stages is usually lower than the other low tide point. This difference is called the diurnal inequality of the tides.
In a few locations, tides occur only once a day, with a single high tide and a single low tide. Such tidal cycles are known as diurnal (daily) tides. In both diurnal and semidiurnal settings, a rising tide is termed a flood tide and a falling tide is termed an ebb tide. The moment when the water reaches its highest point at high tide (or its lowest point at low tide) is called the slack tide, since the water level is then static, neither rising nor falling, at least for a short time.