With regards to human hearing 0 dB is the threshold of human hearing
With regards to human hearing 0 db is normal conversation
With regards to analog audio 0 dB is a power reference derived by the telephone industry in the 1920s
With regards to analog audio 0 dB is the level of the audio is a completely quiet studio
With regards to digital audio 0 dB is the target loudness level during normal programming
With regards to digital audio 0 dB is when all available digitals bit available for audio information are all at maximum value