​OILLETS (from an O. Fr. diminutive of (œil, eye, in Mod. Fr. (œillet; other English variants are oylets, eyelets, or eyelet-holes), the architectural term given to the arrow slits in the walls of medieval fortifications, but more strictly applied to the round hole or circle with which the openings terminate. The same term is applied to the small circles inserted in the tracery-head of the windows of the Decorated and Perpendicular periods, sometimes varied with trefoils and quatrefoils.