Why Do the Two Outer Panes of an Airliner Cabin Window Have a Piece of Cylindrical Metal Separating Them?

Airline windows typically comprise three or more layers of glass, or acrylic, to provide insulation from the very cold atmosphere at altitude.

The tiny silvery cylinder is really the edge of a small hole drilled in the middle layer to allow the pressure to equalize between the layers while minimizing convection.

The condensation around the hole is due to the inner airspace cooling. Ice often forms here.

The position of the hole is chosen to maintain the best clear viewing area when condensation forms.

It also helps to minimize the likelihood of a crack forming between the hole and the edge of the window, and to avoid excessive condensation pooling over the hole, which could freeze and block it.