The ratio of users who successfully click on a specific link, compared to the total number of users who have received or seen the link, often used to measure the relative success of a Call to Action link on a website, other online marketing and the like.
You can technically calculate a CTR on every link on your website, but choosing which ones to monitor is important. You don't want to miss valuable data, but you also don't want to receive so much data it becomes meaningless.