Short for customer-to-customer or consumer-to-consumer meaning business activities between two customers usually involving ecommerce interface or technology.
Although transactions between two customers does not necessarily mean they are conducted online, however, c2c and similar terms are considered to be resultant of growing influence of internet and business conducted in ecommerce environment.
C2C transactions usually involve one customer selling an item to another using third party services that help sale through auction, classifieds/ads etc e.g. OLX, eBay etc.