Albert Einstien once defined it fairly simply (during a college class discussion, I believe).
He simply said (roughly), "Darkness is the absence of light". Another way to put it is that darkness is not a property, but light is. Darkness is simply the absence of this light.
His definition works, both physically (in physics), and psychologically/spiritually/emotionally.
Spiritually, the idea is that "those with darkness in their hearts", simply "have not seen the light".