Chiaroscuro is an Italian artistic term used to describe the dramatic effect of contrasting areas of light and dark in an artwork, particularly paintings. It comes from the combination of the Italian words for "light" and "dark."
Nowadays chiaroscuro applies to a far wider array of dramatic lighting effects, particularly in movies — anything dark and moody with great slashes of shadow, is very chiaroscuro. Film Noir, of all the film genres, is the best example of chiaroscuro. Chiaroscuro remains essentially a technical word, though, and is not often used in everyday conversation — unless you happen to be a film critic of course.