It’s usually called a cable tidy box or extension cord concealment box. If it’s meant to cover the outlet area more like a flush plate, people also call it an outlet concealer or socket cover.

Common terms:

  • Cable tidy box.
  • Extension block cover.
  • Outlet concealer.
  • Socket cover.

For shopping, “cable tidy box” is the most common phrase in product listings, while “outlet concealer” is a good term if you want the plug and cords hidden close to the wall.