What is an impression?

I often find myself explaining what exactly is an "impression" in online-advertising.
I tried to look up a definition on the web but could not find one that I feel describes an impression. All I could find was a definition of an impression as a measuring unit to count advertising dollars (http://bit.ly/mJIYlT), and that, in my opinion, missed the whole point. It is like saying a head is a measuring unit for people (i.e. "head count") whereas a head is first and foremost a... head (eyes, ears, nose, hair - to the lucky ones etc...), which can also be used to count the number of people in a group.

So, what is an impression?
An impression is a combination of a placement and a person, looking at this placement (in a specific time).

...And, a placement is...?
A placement is a space on a web page in which an ad is displayed.

Thus, a single page can generate several impressions for a single time a user opens it (assuming this page has several placements thus showing several ads).
In online advertising we try to create a match between an impression and an ad, meaning, we try to match an ad to a person, browsing the web in a specific location (page) on a specific time.