The click-through rate (CTR) is a metric used to measure the effectiveness of online advertising and email campaigns. It is calculated by dividing the number of clicks an ad or link receives by the number of times it is shown, then multiplying by 100 to get a percentage. A higher CTR indicates that more people are engaging with the content. For example, if a banner ad is displayed 1,000 times and receives 50 clicks, the CTR would be 5%. Marketers use CTR to assess how well their advertisements are performing and to make adjustments for better engagement.