The term "Unlock Key" refers to a device or code that is used to grant access, typically by unlocking something such as a door, safe, or computer file. Full definition
The volume rocker and power / unlock keys sit flush to the device and, as you'd expect, are quite difficult to locate without looking first. (knowyourmobile.com)
Rather than containing actual game data, the cards are just unlock keys to gated content already in the game. (gonintendo.com)