The opposite of light. We can see in light, we are unable to see in true darkness.
The receptors in our eyes work because the photons in light strike the light sensitive cells at the back of our eye and set off an electrical signal which is sent down the optic nerve into our brain. The brain then interprets the signals and creates an image in our mind that represents the reality we see.
In relative, and in artistic, terms we also see “darkness” as areas of lesser light. In fact, because we can see in these diminished light conditions, we are in shadow not darkness – there is some light present. Nevertheless we refer to this sort of “diminished” light situation as “darkness” for dramatic reasons and for ease of communication. The term “darkness” should therefore not be assumed to mean the same as absolute darkness. Instead it should be considered a relative term in which the alternative light conditions may be taken into account.
From the point of view of a photographer, darkness was traditionally assumed to be a light level too low to create a reaction in a light-sensitive photographic chemical gel. The modern cameras, especially DSLRs, are much more sensitive to low light intensities then photographic gels of the past. With certain types of digital image sensor some cameras can collect light of sufficient intensity to create an image although there is insufficient light for the eye to see.
For practical purposes the inability of the human eye to distinguish contrasts, and therefore shapes, would seem to be an appropriate definition point for the term “darkness” to be used in most everyday situations.