Definition
A standardized vertical reference equal to the average height of the ocean's surface, calculated from long-term tide measurements and used as the zero point for measuring altitude. Altitudes given in feet MSL are heights above this fixed reference, regardless of the terrain directly below the aircraft.
Plain English
An imaginary flat surface set at the average level of the sea. When an altitude is given in feet MSL, it means that many feet above that surface, not above the ground you happen to be flying over.
Context Anchor
You will see MSL in airport elevations, chart altitudes, weather information, and before-takeoff checks when comparing the altimeter indication with the published airport elevation.
Derivation
"Mean" here means "average," not "intermediate" or "unkind." Sea level varies constantly with tides and waves, so the reference used in aviation is the long-term average — a stable, agreed-upon zero line that works the same everywhere.
Why Pilots Care
Using MSL altitudes keeps navigation, terrain clearance, and traffic separation consistent on every chart and in every airspace.
Grounding Statement
If an airport is 620 feet MSL, it is about 620 feet above the average level of the ocean.
Intuition Check
MSL does not mean height above the ground under you. It means height above the average sea-level reference.
Example Sentence 1
Tower cleared the flight to climb and maintain 8,000 feet MSL.
Example Sentence 2
Cruising at 4,500 feet MSL kept the flight well above the highest terrain along the route.