Bleed
The extra margin of artwork extended beyond the final trim edge — typically 3 mm or 0.125 in — so that cutting variance never leaves a white sliver at the edge of a printed piece.
Also called: print bleed
Commercial printing trims many sheets at once, and the blade drifts by a fraction of a millimetre. Bleed solves it: you run any background colour or image past the trim line (usually 3 mm / 0.125 in), so even a slightly-off cut still lands on artwork rather than paper. A matching safe area inside the trim keeps text and logos away from the edge.
Bleed is a print concern rather than a screen one, but it matters for the deliverables a mockup often previews — business cards, packaging, stationery — alongside enough resolution at final size.