Cost-per-action is what an advertiser pays for each visitor that takes some specifically defined action in response to an ad beyond simply clicking on it. For example, a visitor might visit an advertiser's site and request to be subscribe to their newsletter.

Payment model whereby payment is based on qualifying actions such as a sale or membership registration by a visitor.

Pay-for-performance model, based on actions taken at the website, usually sales

Advertising payment model based on specific actions of the user in response to an ad. Actions may include a sales transaction, a customer acquisition or a click.

Online advertising payment model where payments are made based on the advertisers desired action (for example a sale or registration)