Plain English Meaning
A made-up but agreed-upon 'average' atmosphere that every country uses as a yardstick. Real conditions are then described as warmer, colder, higher, or lower than this standard.
Full Definition
An internationally agreed model of the atmosphere, published by the International Civil Aviation Organization, that defines fixed reference values for pressure, temperature, density, and lapse rate at every altitude. At sea level it specifies a pressure of 29.92 inches of mercury (1013.25 hPa), a temperature of 15 degrees Celsius (59 degrees Fahrenheit), and a temperature lapse rate of approximately 2 degrees Celsius per 1,000 feet up to the tropopause. It is used as the common baseline for calibrating altimeters, certifying , and comparing real to a standard.