One page impression (PI) is the numerically equivalent of one click on a single site of a website. If one single user (using the same technical device) uses to click on one single site for several times, not every of those clicks counts as one page impression. The average number of PIs per month is important for advertisers (and for their clients).

Advertisers tend to use the number of unique users (UU) to estimate and measure the significance of websites. That means that every IP-adress is counted once in a period of time. It`s not decisive how often this ip-adress visits this special website.