Clickthrough rate (CTR) is a measure of the effectiveness of an online advertisement or email campaign. It is calculated by dividing the number of clicks on the ad or email by the number of times it is shown (impressions), expressed as a percentage.
For example, if an ad is shown 100 times and receives 10 clicks, its CTR would be 10%. A high CTR is generally seen as a positive indication that the ad or email is effective and engaging, while a low CTR may indicate that the ad or email is not relevant or interesting to the audience.