An element that prompts a user to carry out a desired activity
An interface element that aims to encourage users to continue down the optimum path, as identified by the developer. For a website design, example actions would be submitting contact details or making a purchase.
The effectiveness of a call to action (CTA) is often measured by calculating the conversion rate, which at its simplest would be the total number of times the call was successful divided by the number of times the CTA was seen.