Impression is the number of times a piece of content (like an online advertisement) is seen.
An impression is when that specific element becomes visible, that could happen right after page load (i.e top banner), on scrolling down (i.e. content banner) or after some sort of action (i.e exit intent popup).
As in most cases, impression alone is a useless metric. What we really want is to combine it with other, probably more important metrics. For example, clicks/impressions in online advertising.
Often a custom feature
Many analytics platforms, like Google Analytics, don’t track the number impressions by default. It has to be done manually, using custom events.