How often your ad is shown. An impression is counted each time your ad is shown on a search result page or other site on the Google Network.

• Each time your ad appears on Google or the Google Network, it's counted as one impression.
• In some cases, only a section of your ad may be shown. For example, in Google Maps, we may show only your business name and location or only your business name and the first line of your ad text.
• However, when someone searches using Google Instant, an impression can be counted when one of these occur:
  • Person begins to type and then clicks anywhere on the page like a search result, ad, or related search
  • Person types a search and then clicks the "Search" button, presses Enter, or selects a predicted query from the drop-down menu
  • Person stops typing, and the results are displayed for a minimum of three seconds
• You'll sometimes see the abbreviation "Impr" in your account showing the number of impressions for your ad.

* Nguồn: Google