Definition
A standard vertical reference based on the average level of the ocean's surface, used as the zero point for measuring altitude. Altitudes given in MSL describe how high something is above this fixed sea-level reference, regardless of the terrain directly below.
Plain English
A common starting line at sea level that everyone measures height from, so altitudes can be compared the same way everywhere.
Context Anchor
Seen in accident reports, weather reports, charts, airport elevations, and altitude descriptions.
Derivation
"Mean" means average, and "sea level" is the surface of the ocean. The average is used because the ocean rises and falls with tides, so a single fixed reference is needed. This gives every pilot, controller, and chart maker the same zero point to measure from.
Why Pilots Care
Allows consistent altitude references for navigation, obstacle clearance, and reporting regardless of location.
Intuition Check
Do not read MSL as height above the ground. MSL is height above the sea-level reference; height above the ground is a different measurement.
Example Sentence 1
The pilot leveled off at 4,500 feet MSL for the cross-country leg.
Example Sentence 2
The NTSB report listed both aircraft altitude and terrain elevation in feet MSL.