what is an impression in google webmaster tools?

[Jeff]

I know what an impression technically is, but do they mean:

1. The site showed up on the viewers screen? (sometimes this would mean the viewer went through multiple pages of search results)

or

2. The site came up as a search result on page 5 and the user never navigated to page 5, but it was there nonetheless.

Which one counts as an impression in google webmaster tools?

[Richard]

The number of times pages from your site were viewed in search results, and the percentage increase/decrease in the daily average impressions compared to the previous period. (The number of days per period defaults to 30, but you can change it at any time.)