Take a look at the image below from the game Warcraft 2. There's a darkened area (like a fog) that represents the limits of the 'field of view' -- to the effect that you would not see any enemy characters in the fog.
How would you implement this 'fog' in iOS (SpriteKit, Cocos2d, etc)? Mostly interested in hearing high-level strategies on implementing the fog itself -- within the node-based hierarchy of something like SpriteKit or Cocos2d.
And bonus: what do people call this 'fog' in the game industry?
[Update] 'Fog of War' (thanks @Jon)