The impression is the same as a web page counter. The purpose of the impression is to tell the site owner how many times their site has been accessed and in what way.
Each time a page is accessed an impression is made, each time a link is clicked on an impression is made. Page impressions are called hits. This is important in web site advertising where the cost is based on the click through rate, or number of times that an ad is accessed.
This would be determined through counting the number of impressions. This can also be called a view or an ad view if it is with regard to marketing impressions.