Ephemeral film, as defined by film archivist Rick Prelinger, is film made for a specific purpose other than as a work of art: the films were designed to serve a specific pragmatic purpose for a limited time. That is, the genre excludes most well-known film genres such as western film and comedies, and is composed of e.g. advertising films, educational films, industrial films, police training films, social guidance films, government-produced films, home movies and amateur films, among others. Prelinger estimates that the genre includes perhaps 400,000 films and, as such, is the largest genre of films, but that one-third to one-half of the films have been lost to neglect. Many ephemeral films are also grouped under the term "orphan films," since they lack copyright owners or active custodians to guarantee their longterm preservation.

The films are often used as b roll in documentary films, for instance the social guidance film The Terrible Truth appears, desaturated in Ron Mann's film Grass as an example of what he perceives as hysteria over drug abuse, as well as an example of the slippery slope fallacy.

Prelinger and other film archivists generally consider the films interesting for their sociological, ethnographic or evidentiary value: for instance, a mental hygiene film instructing children to be careful of strangers may seem laughable by today's standards, but the film may show important aspects of society which were documented unintentionally: hairstyles, popular fashions, technological advances, landscapes, etc.

In recent years the archival moving image community has taken greater notice of ephemeral film, and key ephemeral films are now being preserved by specialized, regional and national archives.