The Greek noun gnophos (γνόφος) means darkness, gloom, or thick cloud — particularly the kind of dark storm cloud associated with divine theophany. It appears once in the New Testament (Hebrews 12:18), contrasting the terrifying darkness of Sinai with the glorious approach to Mount Zion in the New Covenant.
The gnophos — the dark cloud of Sinai — represents the terrifying holiness of God encountered under the Old Covenant. At Sinai, the people could not approach; even animals that touched the mountain were to be killed. The darkness and fire were signs of God's unapproachable holiness. Hebrews 12 deliberately contrasts this with the New Covenant approach: believers now come not to Sinai's gnophos but to Mount Zion, to the heavenly Jerusalem, to Jesus the mediator. The darkness has been penetrated by the One who entered it on our behalf.