An impression is one time a unit was served: an ad, a banner, or a widget. The count happens at serve time. It does not mean a person looked at it, and it does not mean they clicked. Viewability, a later idea, tries to add "and it was actually on screen." A plain impression does not include that test.
A pageview is a cousin. Both are loads. An impression is usually the load of a specific slot, and a page can have several. A widget load on MapMyVisitors is closer to an impression of that widget than to a careful pageview of the article, especially if the page can render without the image. The unit the counter uses is described in what a visitor counter counts.
When a network sells "impressions," ask whether the creative had to be on screen. When your own widget count moves, you know the file was requested. You do not know the reader paused on the map.
