The question is: What exactly is a film noir, and how do you know if the movie you're watching is one?
Here are a few of my own guidelines to ponder. If the film you're watching ...
Was shot sometime between 1941 and 1958 Is in black and white Features a hero who has a dark past, or gets involved in shady dealings Makes ample use of shadows, and has an overall dark tone, both thematically and in cinematic terms Has scenes in a roadhouse where the hero drinks black coffee and chain smokes unfiltered cigarettes The hero merely says "Rye," and the bartender pours him whiskey, neat A blond "femme fatale" lures the hero into a scheme that results in a battle to the death There's a fair amount of gun play, and someone inevitably ends up dead. Characters often speak in fragmented sentences, and trade wisecracks Stars Humphrey Bogart, Robert Mitchum, Richard Widmark, Barbara Stanwyck and the like
... then, you might be watching a film noir.
Films noir shouldn't be confused with gangster films, which are a slightly different genre that uses organized crime as its focal point.
In a noir, the hero may have dealings with organized crime, but is essentially a lone wolf. Gangster films tend to focus on the dark side of the American dream. Gangsters essentially share many of the same values of home, family and prosperity, as do honest citizens. But their means of achieving their goals are what leads to their defeat.
Typically, gangsters build a crime empire the way an entrepreneur would set up a legitimate business. They rise to the top because they are more efficient, and more ruthless, than their competitors.
Their downfall inevitably comes when they stray from their Horatio Alger-like roots. It's usually pride before the fall. In the end their empire collapses, and the head gangster dies along with it.
In noir, the hero isn't an entrepreneur. He's an alienated loner, sometimes in an existential crisis and desperate to break whatever shackles are holding him down. Sometimes he's living a more or less balanced life, and is unaware of the dissatisfaction gnawing within him.
When he meets the femme fatale, everything changes. As a couple, their credo is, "Let's be bad together." There's something about the chemistry between the two that leads to crimes each wouldn't have committed alone.
Needless to say, it often doesn't turn out well for the hero. But then again, you wouldn't expect it to.