What is Affinity Marketing?

Legal Definition
Affinity marketing is a concept that consists of a partnership between a company and an organization that gathers persons sharing the same interests (known as an affinity group) to bring a vaster consumer base to the opposite party. The first academic approach of affinity marketing has been provided by Macchiette and Roy in 1992 and they described this notion as a combination of affinity and the marketing ideas. They defined the word Affinity as "an individual level of cohesiveness, social bonding, identification and conformity to the norms and standards of a particular reference group" whereas marketing is described to be the "expectation of benefit for the individual satisfying consumer wants and needs". An affinity group is a group which has a solid connection with a considerable number of consumers and which has the possibility to target them in a much easier way than what can be accomplished by way of ordinary marketing process. Affinity groups in which people recognize themselves may be a charitable organization, a football team, the enterprise they work for, a specific company, an organization they belong to… Thus, Affinity group members may be fans, customers, subscribers, staff members…

Affinity marketing differs from co-branding. The benefits of co-branding partnerships come from the consequences of the association of multiple companies, whereas the benefits of affinity marketing derive from the mental satisfaction to have profited the affinity group.
-- Wikipedia
Legal Definition
Grouping companies or brands to sell goods or services. Loyalty is increased for all parties and there is more market exposure.