Documentary films constitute a broad category of nonfictional motion pictures intended to document some aspect of reality, primarily for the purposes of instruction or maintaining a historical record. A 'documentary film' was originally shot on film stock — the only medium available — but now includes video and digital productions that can be either direct-to-video, made as a television program or released for screening in cinemas. "Documentary" has been described as a "filmmaking practice, a cinematic tradition, and mode of audience reception" that is continually evolving and is without clear boundaries.

In popular myth, the word 'documentary' was coined by Scottish documentarian John Grierson in his review of Robert Flaherty's film Moana (1926), published in the New York Sun on 8 February 1926, written by "The Moviegoer" (a pen name for Grierson).

Grierson's principles of documentary were that cinema's potential for observing life could be exploited in a new art form; that the "original" actor and "original" scene are better guides than their fiction counterparts to interpreting the modern world; and that materials "thus taken from the raw" can be more real than the acted article. In this regard, Grierson's views align with Vertov's contempt for dramatic fiction as "bourgeois excess", though with considerably more subtlety. Grierson's definition of documentary as "creative treatment of actuality" has gained some acceptance, though it presents philosophical questions about documentaries containing stagings and reenactments.

In his essays, Soviet film-maker Dziga Vertov argued for presenting "life as it is" (that is, life filmed surreptitiously) and "life caught unawares" (life provoked or surprised by the camera).

The American film critic Pare Lorentz defines a documentary film as "a factual film which is dramatic." Others further state that a documentary stands out from the other types of non-fiction films for providing an opinion, and a specific message, along with the facts it presents.