A marketing campaign often designed (although not necessarily) as a larger game, challenge, or treasure hunt. Begins life focused at a small, tight core group and spirals its way out in a predictable and controllable fashion into the mainstream consciousness. The eventual goal is to create as much user-generated excitement and subsequent media interest as possible, while only spending marketing dollars on the product itself. First coined by Frank O'Connor of Bungie.
Wow, that spiral marketing campaign was innovative and ground-breaking!