Also frequently referred to as ‘CTR’, it is a marketing term for measuring the success of an online advertising campaign. It is defined as the number of times an ad is clicked on, divided by the number of times the ad is shown to online users (impressions), expressed as a percentage. For example, if an ad delivers 100 impressions and is clicked on once, then the CTR is 1%. The relevance of CTR has fallen in recent times, with marketers now more interested in ‘conversion rates’.