Project Charter

A formal or informal authorization that declares a project will begin and lays out its general goals, the why of your project.

A project charter doesn't get into specifics, instead focusing on the commitment to do the project. It names who is responsible, and points at the broad reasons for doing it at all. Where scope defines what you're building and the project plan defines how, the charter focuses on the fact that there is a project at all. Sometimes it's a formal signed document, while sometimes it can be as simple as an email outlining reasons from someone with authority.