A serrated flange nut, specifically Grade 304 or 18-8, is a type of nut with a built-in flange and serrations on the bearing surface. The flange distributes clamping force over a wider area, and the serrations provide a locking feature to prevent loosening due to vibration. The 304 or 18-8 designation indicates it's made from stainless steel, offering good corrosion resistance.