A call to action (CTA) in display advertising is a message or button that prompts the viewer to take a specific action, such as visiting a website, filling out a form, or making a purchase. The purpose of a call to action is to encourage the viewer to take the next step in the conversion process.

CTAs are often used in display ads to guide the viewer towards the desired action, and they can be effective in driving traffic to a website or landing page. Some common examples of calls to action in display advertising include "Learn More," "Sign Up," "Download Now," and "Buy Now."

It's important to consider the design and placement of your CTA, as well as the language used, in order to make it as effective as possible. The CTA should be clear and easy to understand, and it should stand out from the rest of the ad. It should also be relevant to the viewer and aligned with the overall goals of the ad campaign.