If something is creased, it has a line — or several lines — from being folded, sat on, crushed, or pressed. You can crease paper on purpose when you fold a letter; a shirt becomes creased if you stuff it in a bag. Skin creases too: a forehead creases when someone worries. With clothes, a crease is not always a mistake. Smart trousers often have a sharp crease ironed down each leg on purpose; the unwanted kind is the messy, slept-in kind. The word is everyday, not fancy. British speakers sometimes say they creased up when they laughed so hard they folded over — informal, and a stretch of the same picture.