Here's Dave Zeltserman's (wonderful author) interesting essay on the definition of noir at the Artery site. Zeltserman emphasises what an individual, possibly or preferably sick or at least doomed, does to his or her own life. He argues against Dennis Lehane's social class theory of noir, where the failure of man is explained by the circumstances of life, which one cannot influence easily or not at all. Zeltserman is backed up by Otto Penzler, who says: "Noir is about losers. The characters in these existential, nihilistic tales are doomed."
Zeltserman's is what I should call a right-wing theory of noir. There's been a lively discussion over Zeltserman's essay at this Facebook site, and people like Vicki Hendricks (wonderful author) and Woody Haut (wonderful noir and pulp essayist) have been saying most interesting things. The Facebook discussion is public, so I thought I could link it here and quote it. Here's Woody Haut's comment (after which he says: "Does that make sense?") which, to my mind, illustrates what could be the left-wing definition of noir: "Taking into account society as a whole and the forces at work that produce a noir sensibility. To put it bluntly, social issues inevitably become individual issues. Penzler’s definition is as comprehensive as they come, and easier to digest, but only so far as the individual. On the other hand, that's what noir is invariably about. But, in the end, even though he expresses it in simplistic terms, Lehane’s statement ends up being a deeper concept, if only because the social and the individual can’t be separated."
Further on, Woody Haut says that the psychological struggle (that Zeltserman emphasises) and the social issues can't be separated, both affect each other. The background of many noir stories is minimal and sparse, like in Double Indemnity Zeltserman mentions in the discussion, but I don't think this is not not being about social or class issues. I think Woody Haut nails it when he says: social issues become individual issues. The anxiety of one's place in the society, the urge to move upward, even with the help of violence, the frustration or the anger of what one has become when not wanting to on in the society, these are both social and individual issues. In the right-wing theory of noir, these losers are losers because that's all they can do, in the left-wing theory, the same losers are losers because there's no other possilibity for them in the society, be they rich or poor.