The plot of a screenplay written out in prose form, generally in one to two pages. The story, specifically the action of the story; beginning, middle, and end; very clearly written and meticulously proofread.
The treatment should be very spare and straightforward — it should not contain extraneous emotional or cerebral content nor long excerpts of dialogue. I feel that it is crucial for a screenwriter to write a treatment of their story before they begin writing the actual script to see how it works on the page and how it is balanced to the four act structure, and then also after they’re finished to help market it. Many companies will require a one page treatment sent ahead of time for review before they accept the screenplay.
Also called a synopsis or pitch; not to be confused with a long-form 25 page treatment, or even longer scriptment (a phrase popularized by James Cameron) which includes blocked excerpts of dialogue and can be written moreso in a hybrid screenplay format.
It should also be noted that it is extremely difficult for unestablished writers to sell just a treatment without the accompanying screenplay. Employers want to see your unique concept executed on the screenplay page in your voice before they’ll buy just the idea and general structure in treatment form.