The average number of click-throughs per hundred ad impressions, expressed as a percentage.
It is important to distinguish what a click-through rate does and does not measure. The CTR measures what percentage of people clicked on the ad to arrive at the destination site; it does not include the people who failed to click, yet arrived at the site later as a result of seeing the ad.
As such, the CTR may be seen as a measure of the immediate response to an ad, but not the overall response to an ad. The exception involves ads that display no identifiable information about the destination site; in these cases the click rate equals the overall rate.
Merely getting visitors to a site had value when Web site traffic was generally accepted as a measure of success. The trend towards profitability, along with better tracking tools, has resulted in less interest in clickthrough rates and more interest in conversion rates.
A high clickthrough rate does not assure a good conversion rate, and the two rates may even share an inverse relationship. An advertisement geared towards curiosity clicks will result in fewer sales, percentage-wise, than an advertisement geared towards qualified clicks.