What is film noir as a genre? What features/qualities of a film warrant its inclusion in the genre?

Retrospective essay: 8 pages. It is the chance to look back over the genre, your choice of films and emphases.
What is film noir as a genre? What features/qualities of a film warrant its inclusion in the genre? Make use of any of the films we have seen this term, in part or whole, and any others you have seen that strike you as relevant.
The films that were used in the semester were:
The Maltese Falcon
Cat People
Casablanca
Leave Her to Heaven
The Big Sleep
The Killers
Notorious
Panique/Panic
The Stranger
Body and Soul
The T-Men
Force of Evil