Brief definition of "game designer"

I have been trying to write a description/definition of "game designers" in 50 words or less. This is my latest:

“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.”

(Update: I've substituted "design" for "game" because the latter implies that the designer will create/produce the actual changes in the game, which is unlikely to be the case for a video game: someone else will actually make the modification.)