The rate at which people click on a link such as a search engine listing, sponsored ad or a banner ad.  Clickthrough rate (CTR) is the rate at which people click on a link such as a search engine listing, sponsored ad or a banner ad.  The clickthrough rate is obtained by dividing the number of users who clicked on an ad by the number of ad impressions.

Clicks / Impressions = CTR

The ratio between the number of times your ad has been displayed and actually clicked on. Click through Rate is usually expressed as a percentage.