How can we define rarefied gas?

Question

I am studying thermal physics where I come across a term " rarefied gas" . I am not able to understand the meaning of rarefied gas. Can you help me to define rarefied gas?

Answer 1

Rarefied gases are gases under a pressure less than the atmospheric pressure. I'm not sure how they are used to study thermal physics, but under such conditions, it makes sense that the extremes of the molecules of the gas would become more distinct. This would modify the fluid nature and the thermal conducting nature of the gas- by modify I mean that they would differ from that under standard atmospheric conditions since the gas would no longer act like a continuum.