A film theory in which the director of a film is considered to be its primary creative force. Derived from the French word for "author," auteurism got its start in the French cinema journal "Cahiers du Cinema," which promoted various directors as true authors of film -- much as we'd talk about "Charles Dickens' 'David Copperfield'" or "Beethoven's Fifth Symphony," Cahiers favored talking about "Alfred Hitchcock's 'Rear Window'" or "Orson Welles' 'Citizen Kane.'"
Film critic Andrew Sarris refined the theory by ranking a number of directors and setting down some principles he believed would define all auteurs. First, he said that an auteur must be technically competent. Second, he said that an auteur's personality will manifest itself in his films through recurring stylistic traits. Finally, Sarris said that aspects of an auteur's personality may seep into his films -- these are often not noticed until a number of the director's films are studied. For example, Hitchcock's fondness for suspense and mistaken identities is obvious with even a cursory examination of his movies; his fascination and repressed hostility toward blondes isn't that obvious until you've seen them repeatedly threatened, attacked, and killed in his movies...
Some directors are more clearly auteurs than others: it's very easy to see the trademarks inherent in movies by Quentin Tarantino, John Ford, Woody Allen, John Waters, Martin Scorsese, Tim Burton, or James Cameron, for instance. Others who one might be tempted to think of as auteurs don't seem to fit the criteria: Ron Howard has enjoyed great commercial and critical success as a director, but there is no trademark "look" of a Ron Howard film, and while there is a definite "look" to a movie by Ed Wood, Jr., the bit about technical competence rules him out. And critical acclaim is not a requirement for being considered an auteur -- Roger Corman and Russ Meyer will probably never receive Oscars, but they definitely fit all the requirements to be considered auteurs.
Of course, the auteur theory is not universally accepted -- even some of its proponents don't fully agree with it. One of the biggest problems with the theory is that it credits more importance to the director's contribution than any others, including the writer, the producer, the stars, or even the studio -- face it, a movie produced by Steven Spielberg looks a lot like a movie directed by Steven Spielberg; there's no way a Troma movie is ever gonna look like anything other than a Troma movie; and what's important about "Duck Soup" -- director Leo McCarey or the Marx Brothers? The auteur theory also doesn't stand up well with collaborative efforts: did "Who Framed Roger Rabbit" owe its success to director Robert Zemeckis or to animation director Richard Williams?