A question whose answer is more complicated and subjective than would appear at first glance. Film Noir, like pornography, is difficult to define, but I know it when I see it. :)
While sparing the gentle reader and my typing fingers a lengthy discourse on the subject, for which, for those interested, I recommend the excellent wikipedia article on the subject, I think the genre can be characterized, if not defined by the following criteria, not all of which are necessarily present in a given film. You can't call these rules, since the exceptions outnumber the examples. They must be considered typical characteristics.
- A dark, gloomy world view tinged with mistrust, cynicism, and a sense of the absurd.
- Deliberate dramatic use of light and shadow.
- Moral ambivalence of the major characters, who tend to come to a bad end.
- Low or modestly budgeted - B movies either literally or in spirit.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiqJWKva8MA8eJiIOR10OyeKLoSISGYM64D1OTjoN390EUD6vQkf6iY52BCNvLU9fKjAhFAB4rC_OOqjuCC3ucMiZ6ZqI41UNREI0RC-bmxwiqgYelSMkNPM9-3dLHxqUW-eGkhClAT-16w/s400/767px-DetourPoster1.jpg)
An excellent example of mid-40s noir is Detour (1945), which cost $117,000 to make when the average budget for a Hollywood feature was around $600,000. Even at that modest cost, it came in 30% over budget.