The number of times an advert or similar asset has been shown to a user. As such, it is an important metric in on-line advertising.
It can be measured either in terms of raw impressions (the total number of views) or unique impressions (the number of individual people who have seen the advert). These will generally be different, as in many cases someone will see the same advert several times during the course of a single visit to a web site or in a single browsing session.
However, the number of impressions is not a bottom line metric. It is a useful titbit of information for running a campaign, but by itself it doesn’t really mean much.