Film Noir- french film critics after the war noticed stylistic similarities between a number of different Hollywood films. As John says, most critics take the starting point as being around about The Matese Falcon, and the end point being A touch of Evil. (1957) In most film scholarship it doesn't last to the 60s.

Defined by three things: unease about masculinity- hero is anbiguous and morally uncertain; unease about femininity- dramatic split in female characters- 'femme fetale' v angelic good woman; plot is organised around moral uncertainty and ambiguity. Thus, you can define noir in terms of narrative as well as style.

Style is important too of course- use of shadows and High key lighting makes visuals distitinctive and also emphasises ambiguity of the moral world of the detective.

60s noir- Altman Big Sleep for instance is very different, more about the breakown of the values that made noir possible in the first place maybe.