but what is a short film?

A short film is any film not long enough to be considered a feature film. No consensus exists as to where that boundary is drawn: the Academy of Motion Picture Arts and Sciences defines a short film as "an original motion picture that has a running time of 40 minutes or less, including all credits. Short films are by no means considered mainstream, due to the opportunity which is provided to explore unusual plots, characters etc. They're contemporary non-commercial motion pictures that are substantially shorter than the average commercial feature film. This fact gives you far more freedom on how to structure a short film. You have no restrictions, and so the possibilitiies are endless.