Impression

An impression is one display of a piece of content or an ad — the moment something appears before a user, regardless of whether it is clicked.

An impression is the unit on the seen side against a click, which is the unit on the acted side; that pair is the basis of click-through rate. But the definition of seen is itself tricky: is appearing in the page's code enough, or must it actually come into the user's view on screen? That difference between a raw impression and a viewable impression is what drives two tools to two numbers, and in advertising money moves on exactly that definition. A second point is that an impression alone is a vanity metric; a large impression count with no click or conversion rate only says something was shown a lot, not that it had an effect. In web analytics impressions usually belong to the search and ad world, while a pageview is the real unit of site traffic; the two should not be summed.