What is a treatment?

A treatment is essentially a prose version of your screenplay, although its exact definition can vary from person to person. Usually, treatments differ from outlines in that they look more like short stories where outlines look more like breakdowns or bulleted lists, but this is a bit of a generalization.

Most feature treatments are around 10-20 pages, but they can be longer or shorter depending on what the story demands, and what the treatment is for.

The johnaugust.com library has examples of various pre-script story breakdowns and outlines, including a treatment for the Alaska pilot.