Official Aviation Definition
Altitude in aviation is the vertical distance measured from a specific reference datum, most commonly mean sea level (MSL) or ground elevation (AGL). Pressure altitude is determined using a standard barometric datum of 29.92 inHg (1013.25 hPa), serving as the basis for Flight Levels.
In Simple Words
Altitude is how high the aircraft is flying in the sky.
Real-World Flight Example
The pilot leveled off at an indicated altitude of 8,500 feet MSL after clearing the coastal mountain ridge.