CIMM DEFINITION: An online advertising or marketing technique via digital platforms such as websites that encourages users to pass on a marketing or branding message to other sites or users, creating a potentially exponential growth in the message’s visibility and effect.

2: Any advertising that propagates itself. (Source: IAB)

3: Advertising and/or marketing techniques that “spread” like a virus by getting passed on from consumer to consumer and market to market. (Source: IAB)