Stands for "Click-Through Rate."
CTR is a term used in online marketing that measures the percentage of times an ad gets clicked out of the times it appears. It is one measurement of the success of an advertisement in an online marketing campaign using a Pay Per Click (PPC) model.
Online marketers track the CTR for a campaign as a whole, as well as for each individual ad and keyword. This allows advertisers to see which ads are most effective, then tailor their ads for the most effective messaging.
Search engines that show advertisements often take an ad's CTR into account when determining its relevancy and overall quality score. An ad with a high quality score will appear more often and cost less on a Cost Per Click (CPC) basis, which incentivizes advertisers to continually improve the CTR of their ads.
CTR is also a metric tracked in email marketing, measuring the percentage of readers that click a link in an email. Other measurements for email marketing include the open rate (what percentage of people who received an email opened it) and the bounce rate (what percentage of emails sent were not delivered).