You mean, Noise Floor
Any practical measurement will be subject to some form of noise, or an unwanted signal. And, in audio this may be background noise or in electronics there are often things like thermal noise, radiated noise or any other interfering signals. In a data acquisition measurement system the system itself will actually add noise to the signals it is measuring.
Noise floor limits the smallest measurement that can be taken with certainty since any measured amplitude cannot on average be less than the noise floor.
To make it simple for you, noise floor is the level of background noise in a signal, or the level of noise introduced by the system.
So, what's your thoughts regarding what I said to you previously, Dman?