Through a close semantic analysis of the Hebrew terms hata' and hatta't (commonly translated as 'sin') in Leviticus 4-5, this study reveals their lexical meanings, unknown for two millennia both in Judaism and in Christianity. 'Sin' has been commonly understood as referring mainly to a violation of God's commandment pertaining to personal conduct. However the revision Nobuyoshi Kiuchi proposes for the meanings of the terms has significant and far-reaching implications for other major themes such as uncleanness and atonement, meaning human salvation before God. The author also provides a solution to the question of the relationship between sin and uncleanness.