* One who holds the bets in a game or contest.
* One who has a share or an interest, as in an enterprise.
* One who holds the stake above your heart, ready to pound it in. 
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More to the point, the entire list of people with a vested interest in your project.  Often (always?), these are the GoalDonor''''''s.

Projects often fail for political, not technological, reasons.  If the StakeHolders can't agree on the goals for the project, they are likely to all go to the development team to put their own goals in.  This puts the development team in the position of making the business decisions that the business guys refuse to.  At this point, the project has already failed. -- RobMandeville
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Another definition of stakeholder:
* The stakeholder benefits from the company or community.
* The company or community benefits from the stakeholder. -- RonnyClycq