Plain English Meaning
An agreed-upon set of 'average' atmospheric conditions used as a reference. It tells you what pressure, temperature, and air density should be at any altitude on a typical day. Real conditions are then compared to this standard to figure out how the airplane will actually perform.
Full Definition
A model of the atmosphere adopted internationally that defines standard values of pressure, temperature, density, and lapse rate at sea level and with increasing altitude. At sea level the standard values are 29.92 inches of mercury (1013.2 hPa), 15°C (59°F), and a temperature lapse rate of approximately 2°C per 1,000 feet up to the . ISA is used as the reference baseline for calibrating altimeters, , and comparing actual atmospheric conditions to a known standard.