Alternate Reality Game. It's a game or puzzle hidden in the "real" world or the rest of the open internet, often framed as a real-world mystery. Many but not all have their "trail heads" or first clues hidden in a video game. They're often used for marketing or promotional purposes, or to reward fans wtih some extra fun gameplay.

The one I'm most familiar with was the one Valve did to tease the release of Portal 2. One day they pushed an update to Portal whose patch notes included a bizarre line about FCC frequency compliance. Jump in game, and...if you've played the game, you know the radio playing the Muzak version of Still Alive at the beginning? Well there's one of those in every test chamber now, and they've all got a glowing red light on them. If you pick it up and carry it around, you might find somewhere in the room where the light turns green and it starts making a strange sound. Sometimes it's morse code, sometimes it's a weird warbling sound, which turns out to be slow-scan television. Put these images together, and it pointed to a phone number, which if you called responded with modem tones. Players realized that it was an old school bulletin board system, which contained weird fluff text and ASCII renditions of concept art.