ARG is an abbreviation which, in this context, stands for Alternate Reality Game.
An ARG usually uses real-world clues to take players on a journey outside of any traditional entertainment medium. This often involves a chain of leads which is advanced through based on investigative work by participants. In the majority of cases, anyone with an interest should be able to participate. It is common that an ARG will be stumbled upon accidentally and there will be no direct confirmation by its organiser (or 'puppetmaster'), however clues along the way should confirm its existence.
In the context of video games, ARGs often include information surrounding a game or its plot. Sometimes certain clues can only be found in-game, or 'obscure' websites.
Although purists may argue that all ARGs must follow very strict guidelines to be referred to as such, the term is commonly used to describe any cross-media meta game through which players progress based on solving a series of obscure clues.
List of ARGs
Main Article: List of Investigations