MSL in aviation stands for mean sea level, which is a global measurement used to determine an aircrafts true altitude or elevation above the average level of the worlds oceans. It is a constant measurement that pilots can use to ensure they are at the correct altitude during navigation. MSL is used as a reference point when reading aeronautical charts or communicating with Air Traffic Control (ATC) because it establishes a consistent reference point across different regions. On the other hand, AGL (above ground level) refers to the literal height above the ground over which an aircraft is flying. It is used when the aircraft is flying at relatively low heights, such as during landing at an airport, and the altimeter reading becomes less accurate as the plane increases in altitude.