the ratio between the number of clicks and conversions occurring from those clicks.

Conversions/Clicks = Conversion Rate

Conversion rate is the rate at which visitors get converted to customers or qualified leads.  Measuring your conversion rate should be high up on your priority list when measuring the success of your online campaign.  Conversion rate is a measure of a site's ability to persuade visitors to take a desired action, one that you as a site owner want them to take. Conversion rate is a reflection of the site's effectiveness and a measure of customer satisfaction.

Conversion rate can be measured in numerous ways, however a standard formula would be: the number of visitors who took the action you wanted on your site divided by the total number of visitors.  This illustrates a site's overall effectiveness rating for getting visitors to fulfill your goals within a single visit.