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.
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.