A preview window is a small section on a screen that shows a glimpse or preview of something before you decide to open or view it fully. Full definition
When exploring a complex document a small preview window appears on the top left hand corner, which contains a snapshot of the page you are on. (goodereader.com)
While finding that sweet spot, there is a small preview window of text to serve as your guide. (goodereader.com)
In this version a very small preview window appears over the text editing tools that gives you a very small indication of how your changes will look. (goodereader.com)