sea level

water level, average height of the water in the open seas, mean level of the sea's surface

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...

Noun
1. level of the ocean's surface (especially that halfway between mean high and low tide); used as a standard in reckoning land elevation or sea depth
(hypernym) water level

Adjective
1. lying below the normal level; "a low-lying desert"
(synonym) low-lying
(similar) lowland

The level of the surface of the sea; any surface on the same level with the sea.
  

Height of the surface of the sea at any given time.