Level of the ocean's surface, esp. that halfway between mean high and low tide; used as a standard in reckoning land elevation or sea depth.
Average height of the surface of the oceans and seas measured throughout the tidal cycle at hourly intervals and computed over a 19-year period. It is used as a datum plane from which elevations and depths are measured.
Factors affecting sea level are (1) a combination of naturally high tides and storm surge, as sometimes happens along the low-lying coasts of Germany and the Netherlands; (2) the water walls created by typhoons and hurricanes, such as often hit Bangladesh; (3) underwater upheavals in Earth's crust which may cause a tsunami; and (4) global temperature change melting polar ice caps.