Definition: a pay-for-performance pricing model where advertising (such as banners or paid search engine listings) is priced based on number of clickthroughs rather than impressions or other criteria. Overture is an example of a search engine which charges advertisers on a pay-per-click basis
FirstClickSEO Commentary: An important part of most on-line marketing campaigns. But, PPC clicks are only 1 out of 7 vs. 6 out of 7 for organic clicks.