Define Cost Per Action (CPA) in the context of marketing.

Cost Per Action (CPA) is a pricing model used in digital marketing where advertisers pay a fee based on a specific action being completed by a user as a result of their advertisement. Unlike other pricing models such as Cost Per Click (CPC) or Cost Per Mille (CPM), where advertisers pay for ad clicks or impressions, CPA focuses on actions that are more closely tied to desired outcomes, such as conversions or acquisitions.

In CPA marketing, the action can vary depending on the advertiser's goals and objectives. Common actions include:

1. **Lead Generation**: Advertisers pay for each lead generated, such as a user filling out a contact form, subscribing to a newsletter, or downloading a whitepaper.

2. **Sale or Conversion**: Advertisers pay for each sale or conversion generated, such as a user making a purchase, signing up for a service, or completing a registration form.

3. **App Install**: Advertisers pay for each app installation generated by their advertisement, typically used in mobile app marketing campaigns.

4. **Form Submission**: Advertisers pay for each form submission completed by a user, such as a request for more information or a quote.

The key advantage of the CPA model is that advertisers only pay when a specific, predefined action is completed, making it a more performance-based and cost-effective pricing model. It aligns the interests of advertisers and publishers by focusing on measurable results and desired outcomes. Additionally, CPA campaigns can be optimized for maximum efficiency and return on investment (ROI) by targeting audiences more likely to take the desired action and optimizing ad creatives and landing pages to improve conversion rates.