Sea level refers to the vertical change in the height of the sea surface which occurs over all time and space scales from many different mechanisms (including waves, seiches, tides, storm surges, tsumamis, etc.), with tides being the most predictable and the dominant component of sea level variability in many parts of the world’s oceans and coasts. The term water level is used more specifically to refer to the height of the sea surface above some reference level or benchmark (i.e. a tidal datum).
At any particular time or location, the observed sea level can be regarding as being the combination of four main factors: waves; astronomical tides; non-tidal residual; and mean sea level. The term still water level refers to the average water level at any instant, excluding local variation due to waves and wave set-up, but including the effects of tides, storm surges and mean sea level.
Tides are the regular rise and fall of the sea caused by astronomical forces. The non-tidal residual is the part of the sea level that remains once the astronomical tidal component has been removed. This primarily contains the meteorological contribution to sea level, often called the surge. Mean sea level is the average height of the sea over longer periods of time, resulting from changes in ocean volume or mass and/or vertical land movements.