This is a very interesting thread. The term wasn't even in use until the late 1940s. The term Film Noir did not exist until applied to certain films of the immediate post WWII era. It had to do with photography (B/W, shadows and angles, etc.), story (point of view, often fatalism or hubris), acting style and many other intangibles.

Film Noir has a pretty elastic definition. The classic period is considered to begin about 1941 (High Sierra, The Maltese Falcon, Out of the Fog among others) and goes a bit past 1960. There are a handful of pre-1941s that are generally accepted as Noirs (M, The Petrified Forest, They Drive By Night are examples). And there are many films made after 1960/61 and/or in color which get shoehorned into the genre.

There are Noirs that crossover into the western genre, Noir comedies and documentary-style (mostly anti-Nazi/anti Communist) and war Noirs. That is, they share a substantial portion of Noirishness in the story or production values.

There are probably over a thousand films that some expert or other considers Noir. Perhaps half that would be consensus Noirs. My own vision of what is a Noir is very conservative (surprise!); it concerns the main character in some sort of jam, mostly from making wrong choices either in the story, or a some point before the story begins, often knowing that s/he is doing the wrong thing.   My favorite example of that is Born to Kill--Clare Trevor is not a bad person, but jealous of her stepsister (I believe that's the relationship) on whom she is dependent for her lifestyle. She meets a man she suspects of murder, but does not turn him over to the police, instead gets involved with him seeing a way to get out from under her sister's largesse. Needless to say, it ends badly.