Conceptually, sea level is the zero height point where the ocean meets the land. Mean sea level (MSL) at a given point is the average of that level when one has taken into account such things as tides and wave action. Traditionally, measurements had to take into account the effect of the 19-year Metonic cycle and the 223-month Saros cycle on the tides. In actuality it is an extremely difficult constant to determine. Mean sea level is not constant over the surface of the entire earth. Mean sea level at the Pacific end of the Panama Canal is 20cm higher than at the Atlantic end.
The determination of sea level must take into account the ellipsoidal shape of the earth, and the effects of local gravitational anomalies. Over an extended period mean sea level will be affected by global warming in at least two ways: the run-off from melting glaciers on land, and the increased volume reflecting the coefficient of expansion of water. Melting icebergs do not affect ocean levels because the weight of an iceberg is the same as the weight of the volume of water that its submerged portion replaces. (See Archimedes' principle). Despite the difficulties, aviators using instrument flight rules must have accurate and reliable measurements of their altitudes above sea level, and the altitude of the airports where they intend to land. That problem can be compounded when the landing site is an aircraft carrier in a gravitational anomaly.
On other planets that lack a liquid ocean, a "mean altitude" can be calculated by averaging the heights of all points on the surface. This altitude is sometimes referred to as a sea level, and is used equivalently as a reference for the height of planetary features.
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UK
In the UK, mean sea level is defined at Newquay in the county of Cornwall.