Applied to internet marketing this is the measure of conversion by a sales team of inquiries or replies to an advertisement, or mail shot. Simplified, an online conversion rate is the ratio of casual contact viewing converted to the publisher or advertiser’s desired content thread. A conversion rate can be simplified with the following:

Conversion rate = Number of goal achievements ÷ visits

Online conversions also relate to the completion of a pre-defined task, such as registering with a website, making a purchase or clicking on an advertisement.

See also:
Unique page views
Unique visitor
Cost per acquisition (CPA)
Cost per click (CPC)
Reciprocal links
Ad rotation
Ad impression
Behavioural Targeting
Contextual Marketing
RSS
E-Marketing
User Session
Deep Linking