In marketing a conversion occurs when a prospective customer takes the marketer's intended action. If the prospect has visited a marketer's web site, the conversion action might be making an online purchase, or submitting a form to request additional information. The conversion rate is the percentage of visitors who take the conversion action.

Successful conversions are interpreted differently by individual marketers, advertisers, and content creators. To online retailers, for example, a successful conversion may constitute the sale of a product to a consumer whose interest in the item was initially sparked by clicking a banner advertisement. To content creators, however, a successful conversion may refer to a membership registration, newsletter subscription, software download, or other activity that occurs due to a subtle or direct request from the content creator for the visitor to take the action.

For web sites that seek to generate offline responses, for example telephone calls or foot traffic to a store, measuring conversions can be difficult[citation needed]. In order to measure online conversions a marketer can use web analytics software. Some online marketing companies, such as Verizon, offer tracking programs tied to a specific phone number that the business registers for receiving web site inquiries

In Internet Marketing, Conversion optimization is the science and art of enhancing a visitor's online experience with an advertiser's offering when they visit a web site or see an online ad - with the goal of transforming the visitor into a customer (wikipedia.com).