best screenplay not based upon previously published material
The Academy Award for Best Original Screenplay is the Academy Award for the best screenplay not based upon previously published material. It was created in 1940 as a separate writing award from the Academy Award for Best Story. Beginning with the Oscars for 1957, the two categories were combined to honor only the screenplay.
See also the Academy Award for Best Adapted Screenplay, a similar award for screenplays that are adaptations.
Read more or edit on Wikipedia