An ideal gas is a gas whose pressure P, volume V, and temperature T are related by the ideal gas law:
PV = nRT
where n is the number of moles of the gas and R is the ideal gas constant. Ideal gases are defined as having molecules of negligible size with an average molar kinetic energy dependent only on temperature. At a low temperature, most gases behave enough like ideal gases that the ideal gas law can be applied to them.
An ideal gas is also known as a perfect gas.