A short film is defined as a film of fewer than 40 minutes in length.

Besides the obvious titular implications, there are certain common qualities that define a short film. The term, which is used interchangeably with the short subject or simply short. It is used in North America to define a film with a length of 20 to 40 minutes. Feature film trailers are often referred to as shorts, but this is incorrect, as these are not technically short films.

Brief History

Originally coined in the North American film industry, early short films were typically intended to precede feature subjects and were often comedic in nature. One prominent example of early shorts would be the Three Stooges shorts.

More recently, a short film has expanded into a genre of its own. Though there is no definite length cutoff for classification as a short film the American Motion Picture Academy defines any film less than forty minutes as a short film. The Internet Movie Database sets the length at forty-five minutes.

These films are often documentaries on subjects that may be controversial or politically sensitive. Social and environmental themes are popular topics in the content. Older or nostalgia ones, often fall in the realm of “short films” by virtue of their length.

North American types are often character-driven in nature. They focus on topics that more commercially viable feature-length movie will tend to avoid. This can be thought of as derived from the longer feature film format, in the same way, that a short story might be considered a condensed relative of the novel. Typically, they require much smaller budgets. These are released on festivals and art-house theater audiences. Because of the smaller budgets and limited risk in short film releases, they are often used as a showcase for young directing talent.

Short Film Popularity

Many DVDs now include shorts as bonus features. Animated feature films shows a short film from the same production crew. This is often with the same characters as the main feature.  

As camera and film-editing technology become more affordable, the short film is growing in popularity, as many amateur filmmakers can now easily produce a short.  Further, internet release and marketing make the dissemination of amateur movies free and easy, and in some cases even more effective than a theater release. 

Many new online film clubs specialize in distributing otherwise lesser-known ones and documentaries. These films might not be available to the general public without a film club or other distributor of specialty cinema.

Outside of North America, the term short film often encompasses projects much shorter than the typical American short.  In some cases, European shorts will last for only a few minutes.  As such, these ofen lack the focus on character and story typical of American short movies, and will instead be used to showcase unique directorial styles of cinematography.

References:
Short Film Central
Manhattan Short Film Festival