I had heard it referred to as a DV panel, but was told it stood for Direct Ventilation so if the main door opens during flight, you open the little window and it makes the main door easier to close because of the pressure.
Not so sure why it's called a storm window, but i've seen in on piper documents so it is an official name. Not sure what help it would be in a storm.
so many uses for a small flap.