What counts as an impression in webmaster tools?

It bugs me and i haven't been able to find a clear answer. Does an actual human being have the see your site in the serps, or does it simply count the times that keyword has been used as a query?
If it's the former, how come you get impressions for keywords ranked on the 4,5,6 page? Does G pluck your site from the depths of their index to show it to someone? To measure CTR maybe?

No, it doesn't mean it's a human being. An impression is something (or someone) typing a phrase into search and then your site is a part of the data set. You could be first or 1 millionth.

Google takes signals and sometimes something will trigger the algorithm to show your site artificially at the top for them to collect data. Could be that there's a lot of social chatter, could be that you suddenly have something new, could be anything. Google also runs experiments... usually hundreds at a time and your site could be in a test data set. When you see people say "I was first and now I'm last overnight" could be just that they were in a test data set or experiment. These experiments are how Google collects data to improve the algorithm and make changes -- even just to the interface.

But, impressions are basically "your site was included in a search result"

oh, and just to add, automating queries is against their ToS, so you can get your IP temp blocked. But people still do it, so that is where I'm coming from when I say it's not necessarily a human.