1. sea level; (surface of) sea
Wikipedia definition
2. Sea levelMean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.
Read “Sea level” on English Wikipedia
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