Since I have a lot of problems using the same terms for the same things consistently, here a shortlist of what is named what. Don’t hesitate to remind me in case I use something wrong or invent something new that already has a term.
Ad impressions are a measurement of loaded ads that are getting displayed by the user browser. Their number can be affected by ad-blockers and bots.
Ad types define the kind of an ad, e.g. a content ad type has a text editor field, but no other options
An ad unit is the ad you create with with Advanced Ads.
A condition about when and where to display an ad in the frontend.
A page impression happens anytime when a page is getting loaded through a browser. When you navigate to a different page or even use your back button, each time you load that page counts as one page impression.
Ad type-specific attributes of an ad, e.g. the content
A condition from the visitor condition of the ad setting that controls who can see an ad.