In 2006, when I began my study into ARGs, they were a relatively new phenomenon. Used, up to that point, for the promotion of films or video games (beginning with The Beast to promote the film A.I. in 2001, and including I Love Bees promoting the computer game Halo 2 in 2004); the first major non-promotional use, as a stand-alone commercial product, was Perplex City (2005-2007) which became the basis of my study.
Somewhat defying a short neat definition let us turn to two of the genre’s community sites to explain what an ARG involves: