Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. These values, however, are very important in many chemical and physical processes, in particular with regard to measurements. Therefore, it is necessary to define standard conditions for temperature and pressure.
In chemistry, the term standard temperature and pressure (abbreviated STP) denotes an exact reference temperature of 0°C (273.15 K) and pressure of 1 atm (defined as 101.325 kPa). These values approximate freezing temperature of water and atmospheric pressure at sea level.
Also in chemistry, the term Standard Ambient Temperature and Pressure (abbreviated SATP) denotes a reference temperature of 25°C (298.15 K) and pressure of 100 kPa. Although there are many variations of the definition, the most accepted one is the temperature and pressure where the equilibrium constant for the autoionization of water is 1.0x10-14.
The Army Standard Metro atmosphere, now used only in ballistics, defines sea-level conditions as 29.5275 inHg of pressure (99.9916 kPa), 59°F (15°C), and 78% humidity. (Ref: U.S. Army Ballistic Research Laboratory, U.S. Army Aberdeen Proving Ground)
The International Civil Aviation Organisation (ICAO) defines the sea-level International Standard Atmosphere (ISA) as 101.325 kPa, 15°C and 0% humidity. These values provide a reference for calculating various aircraft performance figures, such as endurance, range, airspeed, and fuel consumption. When used to calculate performance at any pressure altitude other than sea level, the temperature is adjusted using the prescribed lapse rate which is -6.5 °C/km for the first 11 km.
(Ref: Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488 / Third Edition, 1993)