The function of the hole in a plane’s window

If you've ever stared out the window as your plane takes off, you will have seen a tiny hole in your window. The Federal Aviation Administration explained that the hole is used to regulate air pressure i.e. they are actually used to stop the cabin from de-pressuring. As the plane gains altitude, the air pressure outside drops compared to the regulated air pressure inside the cabin. The difference between the two puts physical stress on the windows - made up of three panes of glass. There's a small air gap in between the middle and outer panes and the hole is actually in the middle pane. The hole is referred to as a "breather" or "bleed hole - and it balances the pressure between the cabin and the air gap.
The outer pane takes the pressure, whereas the middle pane acts as a fail-safe just in case that trusty outer pane fails. The hole has another role though - releasing moisture from the gap preventing the little windows from fogging or frosting over.