Plain English Meaning
An agreed-upon set of 'average' atmospheric conditions that everyone in aviation uses as a common starting point. Real conditions are rarely exactly this, but using the same reference lets pilots, engineers, and chart-makers compare numbers fairly.
Full Definition
A defined model of the Earth's atmosphere used as a fixed reference for aircraft performance, altimeter calibration, and engineering calculations. At sea level, the standard atmosphere assumes a pressure of 29.92 inches of mercury (1013.2 hPa), a temperature of 15 degrees Celsius (59 degrees Fahrenheit), and a temperature lapse rate of approximately 2 degrees Celsius per 1,000 feet of gain up to the .