What is the definition of a perfect gas ?

Hint: One of the four basic states of matter is gas (the others being solid and liquid). A pure gas can be made up of individual atoms (such as a noble gas like neon), elemental molecules (such as oxygen), or compound molecules (composed of a variety of atoms) (e.g. carbon dioxide).

Complete answer:
A perfect gas (also known as an ideal gas) is a gas that perfectly obeys the ideal gas law in terms of physical behaviour (which provides a relationship between the pressure exerted by a gas, the volume occupied by the gas, the amount of gaseous substance, and the absolute temperature of the gas).

A perfect gas obeys Boyle's and Charles' laws as a result of perfectly obeying the ideal gas law. It's worth noting that no known gas has all of the characteristics of a perfect gas. The general gas law provides an accurate description of the behaviour of a real gas (when the absolute temperature is sufficiently high and the pressure is sufficiently low).

The general gas law provides an accurate description of the behaviour of a real gas (when the absolute temperature is sufficiently high and the pressure is sufficiently low). A perfect gas must also follow Avogadro's law, which establishes a relationship between the gas's volume and the amount of gaseous substance present (total number of moles of gas).

Note: Despite the fact that no gas has these properties, the general gas law describes the behaviour of real gases quite well at sufficiently high temperatures and low pressures, when relatively large distances between molecules and their high speeds overcome any interaction. When a gas, or any of the component gases in a mixture, is near its condensation point, or the temperature at which it liquefies, it does not obey the equation.