Noun: Believe it or not, it's a tuck put on purpose into fabric. Pintucks are ideally very small (~1-2mm wide) and linear. Multiple pintucks are put in, usually at regular intervals, to form a pattern of ridges in the fabric. Pintucks can be made by making a very narrow tuck and sewing through it or can be made automatically by sewing with a double needle with heightened upper thread tension. Pintucks were an heirloom method which was used to show off superior sewing skills, but now that they can be made automatically, they're less common.