Wikipedia in english defines as:
those entities within or outside an organization which:
a) Sponsor a project or,
b) Have an interest or a gain upon a successful completion of a project.
c) May have a positive or negative influence in the Project Completion.
But I actually don't like that definition much - besides being poorly written, it doesn't seem to apply well to a social justice context. Anyone up for editing the wikipedia? It's easy! Though of course this is generally a more business-y term, far too often imported into social change work for my taste.