Bleed
Bleed or the bleed allowance is used in printing and prepress to describe the area that is still being printed but which is outside the actual print and is cut off at the end.
Lexicon
Bleedbox
The bleed box comprises the part of the PDF that includes the content with the bleed. For DIN A4 with 3mm bleed all round, the bleed box is therefore 210×297 plus 6mm, i.e. 216x303mm.