Mean sea level.
The level of the ocean's surface. Sea level at a particular location changes regularly with the tides and irregularly due to conditions such as wind and currents. Other factors that contribute to such fluctuation include water temperature and salinity, air pressure, seasonal changes, the amount of stream runoff, and the amount of water that is stored as ice or snow. &diamf3; The reference point used as a standard for determining terrestrial and atmospheric elevation or ocean depths is called the mean sea level and is calculated as the average of hourly tide levels measured by mechanical tide gauges over extended periods of time.
The level of the ocean's surface, especially the level halfway between mean high and low tide.
The level of the surface of the sea, esp. the mean level between high and low tide: used as a standard in measuring heights and depths.