What is Impression?

Legal Definition
An impression (in the context of online advertising) is when an ad is fetched from its source, and is countable. Whether the ad is clicked is not taken into account. Each time an ad is fetched, it is counted as one impression.

Because of the possibility of click fraud, robotic activity is usually filtered and excluded, and a more technical definition is given for accounting purposes by the IAB, a standards and watchdog industry group: "Impression" is a measurement of responses from a Web server to a page request from the user browser, which is filtered from robotic activity and error codes, and is recorded at a point as close as possible to opportunity to see the page by the user.
-- Wikipedia
Legal Definition
A "case of the first impression" is one without a precedent; one presenting a wholly new state of facts; one involving a question never before determined.
-- Black's Law Dictionary
Legal Definition
An image fixed on the mind; a belief. See 22 U. S. 483, 6 L. Ed. 140.
-- Ballentine's Law Dictionary