The Greek noun zophos (ζόφος) means deep darkness, gloom, murky blackness. It appears five times in the NT (Heb 12:18; 2 Pet 2:4, 17; Jude 6, 13). Unlike skotos (general darkness), zophos denotes a thick, oppressive, nether darkness — the gloom of the underworld or of divine judgment.
Zophos carries overtones of the ancient Greek underworld — the murky realm of the dead. In 2 Peter and Jude, it describes the holding place of fallen angels: God cast them into 'chains of gloomy darkness' (zophos) to be kept until judgment (2 Pet 2:4; Jude 6). False teachers are 'waterless springs and mists driven by a storm, for whom the gloom of utter darkness has been reserved' (2 Pet 2:17; cf. Jude 13). In Hebrews 12:18, zophos describes the terrifying darkness of Mount Sinai, contrasting the old covenant's fearsome approach to God with the new covenant's joyful access through Christ.