What is a Stakeholder?
A stakeholder is any individual, group, or organization that can affect, be affected by, or merely perceive itself to be affected by the project. That last clause is the one people miss — perception is enough to make someone a stakeholder, because perception is enough to make them act.
Stakeholders can be positive or negative, internal or external, and identification starts at day one and never stops. The most expensive stakeholder is the one you find in month nine.
Worked example
A wind-farm project's obvious stakeholders: investor, utility, turbine supplier, landowners. The one that stops the project for a year: a coastal-tourism association two towns away that believes the turbines will be visible from their beach. They're wrong about the sightlines — and it doesn't matter, because perceived impact made them a stakeholder while the register said otherwise.