A crease is a small
fold in something.
A crease in paper is the tiny fold left behind after the paper is folded and then unfolded. Origami folders often use creases to mark distances to measure further folds by.
A crease in fabric can be a wrinkle, or a more deliberate thing, a straight line along the fabric created by ironing so that it folds in a particular way to look neater. Also called pleats, as in a pleated skirt or pleated drapery.