Opacity of app windows

For the last few weeks, I've noticed the windows of all my applications become translucent, ie: I can see a 'ghost' of the desktop or whatever is behind them; occasionally also something I have been looking at previously or have open elsewhere appears beneath a new window when I open it (see image).

This is getting really distracting, especially when working with colour and detailed images. To clarify - I don't mean the opacity of the menu bar etc (which can be turned off an on via System Preferences > Accessibility > Display), but the WHOLE window. Including spec below.