Trailhead

The start, or 'first step', of an ARG. This is often in a form of a mysterious clue that appears on a website, in a game or within a community that piques interest and causes an investigation to begin.

Usually a trailhead will feature something out of the ordinary or obviously deliberate (like hidden text in an image) in order to be considered worth attention. A trailhead may exist for a long time before an investigation starts, as it is generally reliant on someone noticing its presence and digging a little deeper.