What is a written proposal to try and get a film project greenlighted called?

A treatment is a detailed document that outlines a film project, including the story, characters, themes, and potential visual style. It's designed to give producers and investors a clear picture of the project's vision and potential. The treatment is typically several pages long and serves as a persuasive tool to get the project greenlit, providing enough information to entice stakeholders about the project's value.

In contrast, a script is the complete dialogue and action of the film, meant for production rather than pitching. An outline usually consists of a brief summary of the plot and structure, serving more as a roadmap for screenwriters than as a standalone proposal. A storyboard visually represents scenes through illustrations or images, which helps in planning shots but doesn't convey the narrative or thematic elements in the same way a treatment does. Thus, the treatment is the most appropriate document for proposing a film project to secure backing and approval.