Why is there a small hole in airplane windows?

Most commercial aircraft cabin windows are made of three plastic panes, each separated by a small air gap. The tiny hole at the bottom of the window is a bleed hole through the center pane to equalize the pressure inside the window unit with the cabin air pressure. By design, this ensures it is the outermost window pane bearing the load of the internal cabin air pressure pushing outwards on the window during high altitude flight. Venting through the hole also prevents condensation fogging up the windows.