What is meant by the noise floor of a receiver?

The noise floor of a receiver represents the absolute minimum signal level that can be distinguished from the receiver's own internally generated thermal noise. Every electronic component generates some amount of random electrical noise at temperatures above absolute zero. If an incoming signal is weaker than this internal noise, it gets completely buried in the "hiss" and cannot be detected, no matter how much amplification is applied. Thus, the noise floor defines the ultimate sensitivity limit of the receiver hardware itself. A common misconception is to relate the noise floor to external atmospheric noise. While atmospheric noise impacts real-world listening, a receiver's "noise floor" is a specific technical specification of the internal equipment, independent of outside conditions.