Well nut

They are often referred to by the proprietary name Rawlnut or Rawl nut, the name by which they are known in the UK.

The bolt is passed through one of the pieces to be fastened and threaded onto the nut from the flanged end.

As the bolt is tightened, friction from the neoprene flange against the piece being fastened prevents the nut from turning.

[1] Well nuts are not particularly strong, but are useful for sealing holes and/or isolating vibrations.

[2] The well nut was originally developed by the United Shoe Machinery Corporation (USMC).