The Greek noun akatharsia (ἀκαθαρσία) means uncleanness, impurity, or moral filth. It combines the alpha-privative with katharos (G2513, clean/pure). In the New Testament it frequently refers to sexual immorality and moral corruption.
Akatharsia appears in Paul's vice lists as a work of the flesh (Galatians 5:19; Colossians 3:5) and is contrasted with the holiness God calls believers to (1 Thessalonians 4:7 — 'God did not call us to be impure (akatharsia) but to live a holy life'). Romans 1:24 describes God 'giving over' idolaters to akatharsia as judgment. The New Covenant promises heart cleansing that the law could only legislate externally — Jesus touches lepers, the 'unclean,' to make them clean (Matthew 8:3), reversing the pollution logic.