The use of psychology, advertising, and framing to manufacturer content designed to be spread “virally” over the internet.

Any attempt to create an advertising campaign that is spread primarily by word of mouth.

Pandering to the lowest common denominator in an effort to shill some otherwise unsellable product.

Other Definitions

Wikipedia,

Viral marketing and viral advertising refer to marketing techniques that use preexisting social networks to produce increases in brand awareness, through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet.[1] Viral marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message voluntarily.[2] Viral promotions may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, or even text messages.

Dictionary.com,

the promotion of a service or product through word-of-mouth by its customers, esp. via the Internet or email; also, this as a marketing technique.

Citations

How To Review Goldigg,

Everyone has heard about the Million Dollar Homepage, and more than a few of you have heard of the Million Dollar Wiki. Both of these projects sought to leverage the dual pillars of cheap advertisement and viral marketing to turn a quick profit and provide us all with a case study on how an interesting concept and some ingenuity can really take off.

Alternate Reality Games Hit The Mainstream,

They are viral. People love finding clues and solving mysteries. A well designed ARG can attract a huge number of participants.

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