darkness: the absence of light, used both for literal nighttime gloom and, in John's writings above all, for the realm of evil, ignorance, and hostility to God that the coming of Christ is driving out.
On its surface this is an ordinary word for darkness, the plain fact of no light. The Gospels can use it that way, for the dark of a night crossing or the early dark before dawn, and it carries the everyday idea of doing something out of sight, in the dark, as opposed to out in the open.
What makes the word remarkable is the weight John gives it. He sets it against the word for light and turns it into the name for a whole condition: the world as it is before and apart from God, where people do not know him, do evil, and resist the truth when it arrives. This is not darkness as a thing equal and opposite to God. In John's telling it is something made, something that fails to master the light shining in it, and something already on its way out now that Christ has come.
The same word does slightly different work in John's letters. There the cosmic picture narrows to a test of how a person lives: to walk in darkness is to keep sinning and, above all, to hate fellow believers, while those who love walk in the light. The not-knowing that the Gospel stresses becomes, in the letters, mostly a result of the hatred rather than the heart of the problem.
Underneath all of this the word means one thing, darkness, and the differences are shades drawn out by the setting: the literal dark of a verse about the time of day, the secrecy of something done in the dark, and the moral and spiritual dark that the rest of John's writing fills with meaning.