crease

A crease is a line or ridge on a surface formed by folding, pressing, or aging. It may appear on textiles, paper, skin, or other materials and can be straight or curved. Creases can be temporary, arising from handling or humidity, or permanent, set by heat, pressure, or chemical treatments.

In textiles, creases occur when fabric is folded or stored. Some creases are desired as design features; others are removed by ironing or pressing. The likelihood and permanence of creases depend on fiber content, weave, and finishing processes such as heat-setting.

In paper and cardboard, creases are folds along which sheets are bent. They are created by scoring or applying pressure with a tool, and crispness depends on paper weight and coating.

In clothing and fashion, creases can function as design elements (ironed creases on dress pants, sleeve creases) or as functional folds (pleats). Production often uses heat and pressure to set permanent creases.

In biology, facial and other skin creases are lines that form with aging, repetitive movement, or genetics. Examples include forehead lines and nasolabial folds.

In origami and geometry, a crease is the fold line that defines how a sheet reflects to form a new shape; the crease is central to the structure and symmetry of the model.