What is the lowest achievable orbit?

Could you please clarify for me, what exactly is meant by the term "lowest achievable orbit"? Are we referring to the closest distance a satellite or spacecraft can orbit around a celestial body like the Earth, without burning up or crashing into it? Is there a specific altitude or range that is considered the lower limit for stable orbit, taking into account factors such as atmospheric density, gravity, and orbital velocity? Additionally, is there a difference in the lowest achievable orbit for different types of celestial bodies, and how does it vary?