σκότος is a general word, meaning darkness in any sense. γνόφος usually refers to darkness that accompanies a storm. ζόφος meant originally the gloom of twilight. It was then applied in classical Greek to the darkness of the underworld, the gloom of a sunless region. The latter meaning seems to be practically the one which the word has in the N.T. ἀχλύς is specifically a misty darkness.