Marketing targeted at individuals sharing common interests that predispose them towards a product, eg, a company marketing a calcium antagonist through a specialist cardiology journal. Also, a campaign jointly sponsored by a number of disparate organisations that are non-competitive but have a common interest. Most commonly Affinity Marketing occurs when industry owners recognise they can effect equity locked in their brand and customer bases, by advertising  products and services which they don’t normally sell. A broader spectrum of commodities usually benefits each industry member marketing wise.

See also:
Marketing
Campaign
Brand image