The difference between the highest signal level recorded and the absolute maximum the system will permit.  

If using a vu meter this may be as much as 14 dB above the 0 VU mark.

Care must be taken with extreme signal levels, especially in the unforgiving world of digital audio.  With the inevitable noise level of analogue systems, one may want to record with greater levels to garner the greatest signal to noise ratio: analogue distortion is also gentler than digital.