A documentary film is a nonfictional motion picture intended to document some aspect of reality, primarily for the purposes of instruction, education, or maintaining a historical record - wikipedia 
Polish writer and filmmaker Bolesław Matuszewski was among those who identified the mode of documentary film. He wrote two of the earliest texts on cinema ''Une nouvelle source de l'histoire'' (eng. A New Source of History) and ''La photographie animée'' (eng. Animated photography).
Frame of PhD Gheorghe Marinescu's science films
- wikimedia
Both were published in 1898 in French (French language) and among the early written works to consider the historical and documentary value of the film. Matuszewski is also among the first filmmakers to propose the creation of a Film Archive to collect and keep safe visual materials.
In popular myth, the word ''documentary'' was coined by Scottish documentary filmmaker John Grierson in his review of Robert Flaherty's film ''Moana (Moana (1926 film))'' (1926), published in the ''New York Sun (New York Sun (historical))'' on 8 February 1926, written by "The Moviegoer" (a pen name for Grierson).
Conference of "World Union of documentary films" in Warsaw: Basil Wright (left), Elmar Klos, Joris Ivens and Jerzy Toeplitz
- wikimedia
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 definition of documentary as "creative treatment of actuality" has gained some acceptance, with this position at variance with Soviet film-maker Dziga Vertov's provocation to present "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.
Documentary practice is the complex process of creating documentary projects. It refers to what people do with media devices, content, form, and production strategies in order to address the creative, ethical, and conceptual problems and choices that arise as they make documentaries.
Documentary filmmaking can be used as a form of journalism, advocacy, or personal expression.