I've usually heard it called a storm window in Piper aircraft, as someone else has said above, and would love to know why!? I've also heard it called DV, and the suggestions that its a "worst case" way to see something outside if the windscreen is obscured for whatever reason... but I think they're having a laugh if they seriously reckon you could successfully fly or, better yet, land an aircraft by peering sideways out of such a small orifice (as the Bishop said to the actress..!)
Andy
