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.
The prefix of IDs (and also classes) given to elements generated by Advanced Ads. By default the ID prefix consists of the first 5 letters of your URL. It can be changed at Advanced Ads > Settings > General > ID prefix.
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.