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In the instructions for Legendary: A Marvel Deck Building game, a way of increasing difficulty is to "Add +1-5 Power to the Mastermind."
What does that mean? My guess is that it increases its Attack (i.e. the number you need to hit the Mastermind with to defeat them), and I could be blind here but I don't see this clarified anywhere in the rules that come with the game.
Unfortunately there isn't an officially maintained FAQ or Errata for Legendary, but from a few pieces of evidence we can confirm your guess is correct; Power is Attack.
Firstly, the only number that could be added to on all masterminds is the Attack.
Secondly, when the designer Devin Low posts on boardgamegeek he uses [POWER] to reference attack modifiers.
and Finally, if you pull apart the official rules PDF this symbol 
is referenced by /Power, leaving the potential for the mixed terminology ultimately being a careless typesetting error
