trailhead

noun

1. The beginning of a trail, often specifically a hiking trail.

   “Take a bus or a cab to the trailhead, and walk a couple of hours to the magnificent, cow-inhabited Tai Long Wan beach.”

2. The initial clue that leads to an alternate reality game.

   “The trailhead can be exposed to potential gamers through social network services, websites, emails, or sometimes non-electronic media.”