A window opening is the open space if the entire window system (including smaller windows and frames) is removed from the wall. Sometimes there can be more than 1 window inside of a window opening. Most people consider the window to be the opening, but technically, a double hung window is 2 windows, a fan or arch can have as many windows as needed. A window opening is a single hole in the wall if all the windows on your house were removed. Here are some examples:
This means that 1 double hung window with 2 pieces is 1 opening, even though there are 2 separate windows together, it’s still 1 window opening.