What are impressions in Google Webmasters?

From docs:

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.)

Can you point out exactly where Buyselladds is mentioning impressions in their case? Because I think they're talking about impressions of the ads they're serving and not what you're looking at in Webmaster Tools. The general concept is the same eg. how many times did people see your ad, but the two numbers wouldn't have any actual relation between them.

Impressions: 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.) (source)

In advertising it has a similar but different meaning.

Impression : According to the "Basic Advertising Measures," from FAST , an ad industry group, an impression is "The count of a delivered basic advertising unit from an ad distribution point." Impressions are how most Web advertising is sold and the cost is quoted in terms of the cost per thousand impressions (CPM). (source)