DIN ISO 4165 power socket with a tight-fitting, tethered rubber cover to protect against moisture and dirt when not in use.
These are sometimes referred to as Powerlet or Hella sockets and are commonly fitted on motorbikes, especially from European manufacturers. This is panel mounted in a standard 29mm diameter panel hole and secured using the plastic nut provided.
Connection is via 2 x 6.3mm blade terminals.