Dieline
A dieline is the flat outline of a folded component — a tuck box, game box, or insert — showing the printer where to cut, where to fold, and where to apply glue. It's usually a separate layer on the print file in a fixed color (often magenta for cut, blue for fold).
Why it matters
You need a dieline the first time you design a tuck box, deck box, or game box. Print services supply a dieline template; your artwork wraps around the dieline shape and must respect every fold line.
Example
A tuck box dieline for a 54-card poker deck is a cross-shaped flat unfolds about 8" × 6". Solid lines mark the cuts; dashed lines mark the folds; a small tab on the side gets glue.