At Stockholm University I've once wrote a dissertation about film noir, covering some 50 films, and the choice was natural since I've been a fan since my mid-teens, and some of my favourite films can be classified as film noir, although some because they are film noir, and some regardless of being film noir.
Film noir is often called a genre but I would disagree with that because noir is too transgressive. You can have gangster noir, science fiction noir, western noir, detective noir and so on. If a film cannot be called a noir because it is set in space or in the wild west, than does this mean that a noir has to be set in the city? Clearly not since several noirs are set in the countryside. No, it can't be space-specific either.
And it's not a style, because if it was, plenty of films by Michael Curtiz would qualify, despite being far from noir, whereas The Big Sleep (1946) would not necessarily be called a noir, with its lack of shadows and spiderwebs. Neither has it got a flashback structure or a voice-over. It, and many other noirs, lack a lot of the stylistic and narrative devices that are often associated with noir, indeed seen as essential to it. In some ways The Big Sleep does look even less as a noir than other films directed by Howard Hawks.
It does however have a certain sensibility, which it doesn't share with other films by Hawks, a sense of dread and doom, of life hanging in the balance. And for me this is one key to film noir. This sensibility, a combination of the above and a certain detached cynicism about the ways of the world. It's an existentialism the comes out of the city and has then spread out, a sentiment saying that the end is near, nobody is innocent and nothing really matters, accept perhaps a man's honour. That is why Robert Siodmak and Fritz Lang are such great makers of noir, because for them it comes naturally, those themes and sensibilities were there with them even before there was such a thing as film noir. And since it is this sensibility that makes film noir what it is, you can have film noirs set in space or at a ranch, or at a battle field. There should be some sense of a compatible visual style, compatible with the themes, but that is a given since most films try to have such compatibility any way. A question is whether film noir works in colour, and arguably it does, in such a late example as Chinatown (1974).
But although I've mentioned Siodmak and Lang, film noir isn't about authorship, it is about a time and a place in history, primarily the 1940s and early 1950s, with some distinct examples both before and after. Of course the scources are from earlier years, the hardboiled detective novels from the 1920s and 1930s, by the likes of Dashiell Hammet, Raymond Chandler, James M. Cain and Horace McCoy, and British writers such as Graham Greene and Eric Ambler.
Because it's a question of sensibilities and general atmosphere, a film is either noir or it isn't, it can't be partly noir. If a few scenes have a noir-ish look, in the conventional sense of stark contrasts between light and dark, and elaborate plays with shadows, that's not enough, or if some scenes are sinister and fatalistic, that's not enough either. For it to be a proper noir, the above mentioned ingredients must be ever present, from beginning to end. When we watch a film noir we are entering a special mindset, with its own rules (not narrative rules, but behaviour rules), and there is no escaping them. Even those noirs that end happily, can only be happy in the fleeting moment.
Film noir is not an entirely American affair, there's a whole bunch of British noirs, and some sprinkled out in other countries as well. Bergman's High Tension aka This Can't Happen Here (Sånt händer inte här) is definitely on noir territory.
But definitions are tricky, and although I took a line of dialogue from The Maltese Falcon and put it on the front page of my dissertation, I'm not 100% sure it is a noir, although arguing that it isn't will only complicate things, since it was one of the first films singled out by the French in the post-war era as being an example of the new style of American filmmaking that they had detected. But on the other hand they did also include Lost Weekend (1945), another great film, but one I would say with much more confidence that it is not a proper noir, even though the cinematography by John F. Seitz is spectacularly expressionistic. But more quintessential examples are Double Indemnity (1944) and D.O.A. (1950). Out of the Past (1947) is another strong example. It has a companion piece in The Big Steal (1949), with the same duo, Robert Mitchum and Jane Greer, in the lead, and the same scriptwriter, Daniel Mainwaring, but that is arguably not a noir, in that it is too playful and bright. That might be partly due to the different sensibilities of the directors, with Jacques Tourneur being a somber shadowmaker, whereas Don Siegel is more of an individualist and free-wheeling anarchist to really accept the sense of inevitability in proper noir. (Daniel Mainwaring on the other hand is a key noir-player, with several important screen writing credits, sometimes under the name Geoffrey Homes.)
In the end there will always be film noirs to argue over, as to whether they are or not, and it isn't important. What's important are the films, and there are plenty of them to go around. Perhaps the best definition of film noir comes towards the end of Chinatown. In the film, Chinatown is not so much a place as a state of mind, and when Jake Gittes (the main character, played by Jack Nicholson) is bewildered, angry and sad his partner says "Forget it Jake, it's Chinatown". You could easily exchange "Chinatown" with the words "film noir".