CPA: this acronym stands for "cost per acquisition" or "cost per action." It refers to an internet advertising model. In this model, the advertiser pays a flat fee or a percentage of the sale for each conversion of a site visitor into a paying customer, regardless of click-through or conversion rates. As a result, publishers typically like this model least, because click-through and conversion rates are often beyond their control.
However, advertisers often benefit significantly from CPA models because the system only requires them to pay for a specified action, such as signing up for a newsletter or even a click. As a result, CPA models can often be designed to meet specific goals and budgets. Some advertising campaigns will reason that any exposure an internet user receives to the service in question can be beneficial; meanwhile, others may want to direct their efforts solely towards converting site visitors into paying customers. CPA models can be set to charge based on the according action.
CPA payment models originated in more traditional forms of advertising techniques: TV and radio stations occasionally use similar methods to sell unsold inventory this way, and print media occasionally will use this pricing model as well. However, the most commonly found examples of CPA pricing models are found online.