What does ultra-rarefied gas mean?

Without using any math, can you explain to me what a rarefied gas is? And then what an ultra-rarefied gas is? I'd like to understand it from a conceptual level if you can make connections to other concepts that would be even better. I'd like to imagine the concept in my head.

I also saw this question which did give me some insight.

• "rarefied" is the opposite of "compressed". It's gas at a low pressure, lower than atmospheric. Nov 21 '21 at 15:50

This only works when the gas particles (atoms or molecules) are locally in equilibrium, and this means they have to be exchanging energy with each other on a timescale and length scale much shorter than the ones used in our experiments. In a gas like air at room temperature and pressure the distance gas molecules travel in between collisions (the mean free path) is around $$0.1$$ microns and they collide with each other roughly every $$\stackrel{1}{}\!\!\unicode{x2215}_{\!\unicode{x202f}3}$$ of a nanosecond. So if we are calculating the air flow over a Boeing 747 it is fine to approximate the air as continuous.