Noise floor
The noise that is still there when nothing is playing — the level below which anything quieter simply disappears.
Every audio chain hisses. Resistors produce thermal noise at room temperature, amplifiers add their own, tape had its trademark hiss, and digital audio has quantization noise: 16 bits puts the floor around −96 dBFS, 24 bits theoretically around −144 dBFS, far below what any real analogue circuit manages. The distance from the floor up to clipping is the dynamic range.
Noise floors are usually quoted A-weighted, which discounts the bass and extreme treble where the ear is insensitive — and happens to make the spec sheet look better. Dither raises the floor slightly but trades distortion for smooth noise, which the ear forgives far more readily. Noise shaping can then push that noise into frequencies where it is harder to hear.