Brief definition of "game designer"

I have been trying to define "game designer" in 50 words or less.  This is my latest attempt:

“A game designer conceives the framework for a series of interesting challenges in the form of a ‘game’, devises mechanics (rules), creates or helps create a working prototype, and repetitively and incrementally modifies the design in the light of playtesting until it is a good game for the target audience.”