Definition
An altitude expressed in feet measured from mean sea level (MSL) rather than from the ground or any other reference. It is the altitude shown on the altimeter when the instrument is set to the current local altimeter setting, and it is the standard reference used in most ATC clearances, charts, and instrument procedures below 18,000 feet.
Plain English
It means the airplane's height is being given as feet above sea level, not feet above the ground below you. So if a chart or controller says '5,000 MSL,' that is 5,000 feet above the ocean's average level, no matter what the terrain underneath is doing.
Context Anchor
Seen on charts, weather reports, airport information, and altitude instructions where a common height reference is needed.
Derivation
MSL stands for 'mean sea level' -- 'mean' here is the average sea level worldwide, used as a fixed baseline. Saying altitude is 'expressed' in MSL just means the number is being stated using that baseline rather than another one (like height above the ground).
Why Pilots Care
Determines terrain clearance, airspace compliance, and accurate altitude reporting to controllers.
Analogy
Think of mean sea level as the zero mark on a very tall ruler. MSL altitude tells you where something sits on that ruler, even if the ground below it is already high.
Intuition Check
Do not read MSL altitude as height above the ground. It means height measured from mean sea level, so a 5,000-foot MSL altitude over a 4,000-foot mountain is only about 1,000 feet above that mountain.
Example Sentence 1
Cleared to maintain 6,000 -- the controller means 6,000 feet MSL, so the pilot sets the altimeter to the local setting and climbs until it reads 6,000.
Example Sentence 2
With the altimeter set to 29.92, the pilot read 4500 feet MSL over the plains.