0 dBFS
Definition
The maximum possible level in a digital audio system. "dBFS" stands for decibels relative to Full Scale. Any signal reaching 0 dBFS has hit the absolute ceiling of the digital format.
In Simple Terms
The absolute ceiling of digital audio — like the top of a glass. Your sound can't go higher than this without clipping and distorting. Always leave some space below it.
In Practice
A limiter is set with a ceiling of -0.3 dBFS to prevent intersample peaks from clipping during streaming playback.
Common Confusion
0 dBFS is not "loud" — it is a ceiling. A track at -6 dBFS can measure louder perceptually (via LUFS) than a track sitting at 0 dBFS with crushed dynamics.