Internet advertising where the pricing is based on a cost per thousand impressions is called CPM. (Where CP = cost per and M is the Roman Numeral for 1,000). You will see it referred to occasionally as CPI (cost per impression) but in popular usage CPM has mostly taken over.
Different sites will have different CPM values to different online advertisers. Programmes such as Google AdSense try to put a value on their publishing members for CPM that is optimal for the programme and acceptable to the customer. Other advertisers will simply make an offer to display their banner on your site for $20 per 1,000 impressions. An impression is simply each time the banner (or other advertisement) is displayed on your site to one of your readers.