From Wikipedia:

An impression (in the context of online advertising) is a measure of the number of times an ad is displayed, whether it is clicked on or not. Each time an ad displays it is counted as one impression.

Counting impressions is the method by which most Web advertising is accounted and paid for, and the cost is quoted in CPM (cost per thousand impressions). (Contrast CPC, which is click- and not impression-based.)

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 – 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