Director and short film expert Genevieve Clay-Smith says,

A short film is exactly as it sounds, it’s a film that: due to the time restraints upon it, usually tells a story within 2 acts, rather than 3 like in most feature films or 4 acts like in some television shows.

Most film festivals define a short film as, any film that plays under 40 minutes, however many festivals these days don’t accept short films into competition that are longer than 15 minutes. Filmmaker, Genevieve Clay-Smith whose films have been screened at over 100 film festivals says:

Film festivals receive thousands of entries each year. They don’t want to program films longer than 15 minutes because they want to screen as many great films as possible and long films eat up their time slots. Furthermore, if programmers are not engaged with the film within the first 5 minutes of previewing it, chances are they won’t watch to the end and it won’t be selected, so it’s important to make your film as short and as engaging as possible. What programmers and film festivals think is important, because it is within film festivals and on the Internet that a short film will find its audience.

While short films can draw on conventions of other genres, they are in their own distinct category of filmmaking and  follow their own set of conventions, language and format guidelines.