Click-through rate (CTR) is a metric that measures the effectiveness of an online ad by calculating the ratio of clicks to impressions. CTR is typically expressed as a percentage, and it is calculated by dividing the number of clicks on an ad by the number of times the ad is displayed (also known as impressions).

For example, if an ad is displayed 100 times and receives 10 clicks, the CTR would be 10%. A higher CTR generally indicates that an ad is more effective at driving users to take action, such as clicking through to a website or making a purchase.

CTR is an important metric for advertisers and marketers to track, as it can help them understand how well their ads are performing and identify areas for improvement. For example, if an ad has a low CTR, it may be necessary to optimize the copywriting, design, or targeting of an ad creative to increase its effectiveness.

In addition to CTR, other metrics that are commonly used to measure the effectiveness of online ads include cost-per-click (CPC) and conversion rate (the percentage of users who complete a desired action, such as making a purchase).