Definition
Altitude expressed as the vertical distance of an aircraft above mean sea level. It is the altitude shown on the altimeter when the instrument is set to the current local altimeter setting, and it is the reference used for most published altitudes on charts, in clearances, and for cruising levels below 18,000 feet.
Plain English
How high the aircraft is above the average level of the ocean's surface, regardless of the terrain directly below it.
Context Anchor
Seen on charts, in weather information, in airport data, and in normal altitude assignments where heights are given using sea level as the reference.
Derivation
MSL stands for Mean Sea Level. 'Mean' here means average — sea level rises and falls with tides and waves, so aviation uses the long-term average as a fixed worldwide reference point. From this single agreed reference, every aircraft and every chart can measure heights the same way.
Why Pilots Care
All en route IFR altitudes and most VFR cruising altitudes are assigned and flown as MSL values to ensure safe vertical separation from terrain and other traffic.
Grounding Statement
An airport on a mountain may already be 6,000 feet altitude (MSL) while the airplane sitting on its runway is still 0 feet above that runway.
Intuition Check
Altitude does not always mean height above the ground below you. Altitude (MSL) means height above average sea level, even when the ground below is much closer than sea level.
Example Sentence 1
Example Sentence 2
ATC cleared the flight to maintain 8,000 feet MSL until reaching the initial approach fix.