Cost per action (CPA) is a pricing model used in online advertising, where the advertiser pays a certain amount of money for each specific action that is taken by a user. This action could be anything that is important to the advertiser, such as making a purchase, signing up for a newsletter, or filling out a contact form. The cost per action is calculated by dividing the total cost of the ad by the number of actions that were taken by users.

Effective cost per action (eCPA) is a measure of the average cost per action that is achieved across an advertising campaign. It is calculated by dividing the total cost of the campaign by the total number of actions that were taken by users. eCPA can be useful for comparing the performance of different advertising campaigns and for making data-driven decisions about how to optimize the campaign to improve its performance.