In online advertising, the term "impression" refers to: the Web site displayed when a user clicks on an advertisement. the measure of a user's perception of ad effectiveness. ads that run before a user arrives at a Web site's main content. each time an ad is shown. the design and content of an advertisement.

Each time an ad is shown

Explanation:

The term view or add view can both be used to refer to impression in online advertising. It is basically a record of the number of times a web visitor sees an advertisment that is displayed on the webpage. To determine this number for a particular advertisement, the number of times the page containing the add is visited and loaded is counted as the advertisement is always going to be shown at each loading of the page.