Stands for "Cost Per Action," and is used in online advertising. CPA defines how much revenue a publisher receives when a user clicks an advertisement on his website and then completes a certain action. For example, a publisher may place a banner or text link from an advertiser on his website. When a user clicks the link, she is directed to the advertiser's website. She might then be asked to fill out a form or take a survey. If she completes the form or survey, the action has been completed, and the advertiser pays the publisher a certain amount based on the CPA.
CPA and CPL (cost per lead) are often used interchangeably, though CPA is more generic.