Definition:
Viral loops are a type of marketing campaign used to promote a company or product. Named after our friends the viruses, a viral loop is meant to infect a person and cause them to share it with other people.
A viral loop is designed to go from zero to thousands or millions of viewers in a very short time. It spreads exponentially, as each new customer or user shares the content with any number of friends.
Further Reading:
Book: The Lean Startup by Eric Ries