The number of visitors who convert (take a desired action at your site) after clicking through on your ad, divided by the total number of click-throughs to your site for that ad. (Expressed as: total click-throughs that convert / total click-throughs for that ad = conversion rate.) For example, if an ad brings in 150 click-throughs and 6 of the 150 clicks result in a desired conversion, then the conversion rate is 4% (6 / 150 = 0.04). Higher conversion rates generally translate into more successful PPC campaigns with a better ROI.