This article explains what an impression is and how THEOplayer calculates impressions.
An impression is counted when a video or audio source is being played. When you pause, play or seek within the same video or audio source, this is counted as only one impression.
As an example, if you start a video, pause the video and then resume again it is only considered one impression. If on the other hand you start a video and then refresh your browser it would be considered 2 impressions.