Google posted the following regarding impressions with Google Instant. I think its worth understanding the difference as this is going to affect your CTR/Quality Score of your account. We also added a screenshot which might make it easier.
When someone searches using Google Instant, ad impressions are counted in these situations:
- The user begins to type a query on Google and clicks anywhere on the page (a search result, an ad, a spell correction, a related search).
- The user chooses a particular query by clicking the Search button, pressing Enter, or selecting one of the predicted queries.
- The user stops typing, and the results are displayed for a minimum of three seconds.
- Google Instant: A More Innovative Approach to Search (adwords.blogspot.com)
- Here’s What People Are Saying About Google Instant (GOOG) (businessinsider.com)
- Google Instant: The Complete User’s Guide (searchengineland.com)