An Impression is one of the standard measurement units used in online analytics software.
Impressions track how many times an application or webpage, or parts of it, is being viewed. The term most often refers to page impressions which is synonymous with page views. An impression can also be ,if the webpage is displayed in the Google search results or when the app is displayed in the app-store list.
However, publishers and advertisers are also interested in measuring unique impressions, which only count the numer of unique visits to an application or website. Unique impressions are usually counted by dropping a cookie on visitors´ browsers that expires after 24 hours.
As opposed to this, total impressions are the total amount of time an application or website is being displayed, meaning if a user is looking at three different pages of an application, three page impressions will be counted.
While impressions typically refer to page views, they may also define how many times individual elements of an application or web page are displayed. For example, in online advertising, ad impressions track the number of times indivual advertisements are displayed. If a webpage contains three ad units, each page view will produce three ad impressions.