MEAN SEA LEVEL

Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level [1], however, involves complex measurement, and accurately determining MSL can prove difficult.Mean Sea Level as used by military Flight Surgeons and Aerospace Units: Using pressure to measure altitude results in two other types of altitude. True or MSL (mean sea level) is the next best measurement to absolute - and in some ways better. MSL tells you how far you are above an imaginary line at sea level. If you then know the elevation of terrain, the next step is to determine how far you are above ground. It also tells you how thin the air is, which determines your physiological response to that altitude. True Altitude (MSL) has been adjusted for local high or low pressure conditions. FL or Flight Level is another related term that is measured in hundreds of feet. At a standard pressure that correlates to 18,000 feet, the flight level is one-eight-zero.
See more at Wikipedia.org...

Sea level can refer to:Sea level --- the average height of the seaSea Level (band) --- the `70's group Sea Level
See more at Wikipedia.org...

The average sea level for a particular geographical location, obtained from numerous observations, at regular intervals, over a long period of time. 1/3/73

Hauteur moyenne du niveau de la mer en un lieu géographique particulier. Elle est déterminée à partir de nombreuses observations faites à intervalles réguliers et portant sur une longue période de temps. 1/3/73

Tidal datum that is the arithmetic mean of the hourly water elevations observed over a specific 19-year Metonic cycle (National Tidal Datum Epoch). Shorter series are specified in the name; that is, monthly mean sea level and yearly mean sea level. See: datum