From Wikipedia:

Cost Per Action or CPA (sometimes known as Pay Per Action or PPA) is an online advertising pricing model, where the advertiser pays for each specified action (a purchase, a form submission, and so on) linked to the advertisement.

Direct response advertisers consider CPA the optimal way to buy online advertising, as an advertiser only pays for the ad when the desired action has occurred. An action can be a product being purchased, a form being filled, etc. The desired action to be performed is determined by the advertiser. Radio and TV stations also sometimes offer unsold inventory on a cost per action basis, but this form of advertising is most often referred to as “per inquiry.”

The CPA can be determined by different factors, depending where the online advertising inventory is being purchased.