Definitions for "Click-through rate"
Percentage of clicks registered on the e-mails sent in a given campaign.
Calculated by dividing the click-through number by the number of ad impressions. CTR has become a major measure for Internet ad-campaign effectiveness and provides a basis for comparison of sites and/or creatives. However, click-through rates are not a complete measure of campaign effectiveness (they do not measure user activity past the initial click or conversions). Click-through rates should be combined with other measures to determine the real value of a campaign.
percentage of people who click through to another site (p. 271)
Keywords:  sent, ratio, mails, total, throughs
Ratio of click-throughs to total e-mails sent.