A Call to Action (CTA) is a prompt on a website that tells the user to take some specified action. It's a part of the webpage that a visitor needs to click which leads them to the next step in the conversion process. CTAs are typically written as a command, such as "Sign Up" or "Buy Now" and often take the form of a button or hyperlink.

In the context of A/B testing and experimentation, a CTA is a critical component as it's often the key to lead users towards the conversion goal. For instance, in the Obama 2008 and 2012 campaigns, the digital team determined a conversion metric like CTA button clicks to measure the effectiveness of different campaign strategies.

Here are some examples of CTAs:

1. "Donate Now": In the context of a political campaign, a CTA could be a "Donate Now" button on a campaign website. The team could then measure the effectiveness of the campaign by the number of times this button was clicked.

2. "Sign Up for Our Newsletter": This could be a CTA used in the campaign's newsletter strategy. The effectiveness of the strategy could be measured by the number of sign-ups.

3. "Start Demo": In a product demo flow, a CTA could be a "Start Demo" button. The number of users who click this button and start the demo could be a measure of the effectiveness of the demo design.

Remember, a good CTA should be action-oriented and create a sense of urgency, encouraging users to take the desired action immediately.