I have a description of the genre, which even if it might not be entirely accurate or always applicable (what is?), I always like.

If a private eye is hired by an old geezer to prove his wife’s cheating on him and the shamus discovers long-buried family secrets and solves a couple of murders before returning to his lonely office – that’s detective fiction. If the same private eye gets seduced by the geezer’s wife, kills the old coot for her, gets double-crossed by his lover and ends up shot to death by his old partner from the police force – I can say with complete assurance: you are wallowing in NOIR."
Add mood music, sprinkle your campaign with elaborate description about ceiling fans and smoke, and give players who do private eye monologue before an act a bonus advantage, and you're golden