Pay Per Install (PPI) is a pricing model in mobile advertising where advertisers pay for each installation generated by their ad campaign. For instance, if Advertiser X bids $1.00 to display their ad to a specific user group, and the campaign leads to 1,000 installs, the advertiser will pay a total of $1,000 ($1.00 x 1,000). The cost is directly proportional to the number of installations resulting from the campaign.