Definition:
A documentary is a non-fiction film that uses real events, real people, and real footage to construct a narrative or argument about the actual world. It is one of cinema's two fundamental modes alongside fiction, and it encompasses an enormous range of forms from observational cinema to personal essay to advocacy filmmaking.
What It Means in Movies:
The documentary form is defined by its relationship to reality rather than by any single technique or structure. Some documentaries are observational, following their subjects without intervention. Others are constructed primarily from interviews and archival material. Others use personal narration to build an essayistic argument. What all share is a commitment to the real as their source material, and to the challenge of shaping that material into something an audience can follow and be affected by.
Why It Matters:
Documentary matters because it is one of the primary ways cinema engages with the world beyond the screen. The best documentaries change how audiences understand real events, real people, and real systems. They are also formally inventive in ways that fiction sometimes cannot be, because the constraints of working with reality as it exists force filmmakers to find creative solutions that a scriptable situation would never produce.
Example:
Errol Morris's The Act of Killing invited perpetrators of the Indonesian mass killings of 1965 to re-enact their murders in the film genres they loved. The resulting documentary is unlike anything achievable through conventional documentary methods, using the subjects' own self-mythologising to expose the psychological reality of atrocity in a way that straightforward testimony could not.