Trying to ascertain what a stakeholder definition is, is a tough task for the simple reason that there are so many different types of stakeholders, depending upon the project, venture or enterprise involved.
However understandng who is a stakeholder is a good start. From this we can deduce that at a high level, a stakeholder definition is one who makes the decisions and gains a benefit from a successful outcome; ie someone who has a stake in it. By this I mean someone who has either invested their time, money or ideas into an enterprise.
However once you start looking at the detail you realise that this is too narrow a definition. Hence let's have a look at the different types of stakeholders to better gain an understanding of what their role is.
Sponsors are people with power who have a direct interest in the project. As described elsewhere, you need to carefully manage your sponsors to gain support and avoid opposition, especially where they are also Targets.
There are four types of sponsor:
Initiating sponsor : Who kicks off the project.
Key sponsor : Who provides ultimate authority.
Primary sponsor : Who must be centrally engaged.
Secondary sponsor : Who needs to be kept informed.