"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.)"
Pretty easy to understand; impressions could be explained as "the number of opportunities our site had to get a click in a Google SERP".
My question is, is this calculated by SERP (your site appears in a SERP, you get 1 impression) or by URL (a URL of your site appears in a SERP, you get 1 impression)? I ask this because I have doubts of what happens if, for a search query, your site appears with multiple URLs; for example, if you search for "new york times" (http://www.google.es/search?q=new+york+times), you get a SERP with the first search result being the webpage of the New York times and under that 6 sitelinks to internal sections of that webpage. So, in this case, 7 URLs are showed in the SERP; will this count as 1 impression for the keyword "new york times", or 7?
This affects a lot to the way of analyse that data, also if you see impressions by URL.