What is an advertisment
impression? An advertisement's
appearance on an accessed
Web page. For example, if the
page you're on shows three ads,
that's three impressions.
Advertisers use impressions to
measure the number of views their
ads receive, and publishers often
sell ad space according to
impressions. (It can be tough to
know, though, whether an impression
really means a visitor saw the ad,
since they could be
browsing without graphics or
might not have
scrolled down far enough.)
Impressions are tracked in a log
maintained by a
site server and are often sold
on a cost per thousand (CPM) basis.