It is a noun. The definition of a conversion is a change from one religion to another or adopting a religion. An example of conversion is a Catholic switching to Taoism. A conversion is defined as an exchange from one unit of measure to another. An example of conversion is exchanging dollars for euros.
In electronic commerce, conversion marketing is marketing with the intention of increasing conversions--that is, site visitors who are paying customers. The process of improving the conversion rate is called conversion rate optimization.
