Wikipedia
A flange nut is a nut that has a wide flange at one end that acts as an integrated, non-spinning washer. This serves to distribute the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface. These nuts are mostly hexagonal in shape and are made up of hardened steel and often coated with zinc.
The flange may be serrated to provide a locking action. The serrations are angled such that they keep the nut from rotating in the direction that would loosen the nut. Because of the serrations they cannot be used with a washer or on surfaces that must not be scratched. The serrations help in preventing the vibration of the nut from moving the fastener, thus maintaining the holding power of nut.
The flange nuts are sometimes provided with a swivel flange which helps in creating a more stable structure without affecting finished product like a serrated flange nut. Swivel flange nuts are mostly used in joining wood and plastic. Sometimes both faces of the nut are serrated, permitting either side to lock.
The following specifications define flange nuts: DIN 6923 (superseded by DIN EN 1661) DIN EN 1661 ISO 4161