The Greek numeral hexakosioi means six hundred. It appears in Revelation 13:18 as part of the famous number of the beast: 'six hundred sixty-six' (hexakosioi hexekonta hex).
The number 666 in Revelation 13:18 has generated centuries of theological debate. John calls for 'wisdom' in calculating it — it is the 'number of a man,' likely a numerical cipher (gematria) for a specific figure (Nero Caesar is the most historically supported candidate). Theologically, 666 is the number that falls short of perfection. Seven is the number of completion in Hebrew thought (the seventh day, seven churches); six is incomplete, one short. Three sixes — triple failure, triple incompletion — is the number of a power that claims divine authority but is fundamentally and irredeemably human and fallen. The contrast with God's perfect number underscores the beast's ultimate inadequacy. Every earthly power that demands ultimate allegiance is, in some sense, '666' — impressive but falling short of the divine.