A gas in which the mutual interactions of the gas particles are negligible, except for their momentary collisions.
An ideal gas obeys the ideal gas law, which is a formula relating temperature, pressure, and volume for an ideal gas. Nearly all real gases obey the ideal gas law to very high temperatures and pressures, even those found in the interiors of stars. The specific relation is
P = nkT
where P is the pressure, n is the number of gas particles per unit volume, T is the temperature, and k is the Boltzmann constant, a fundamental constant of nature relating temperature to energy.