The Greek adjective katharos means clean, pure, or undefiled — free from dirt or contamination whether physical, ceremonial, or moral. It is the root of the English word 'catharsis' (purification).
Katharos spans the full range of purity in the New Testament: physical cleanliness, ritual purity, and above all moral and spiritual purity of heart. Jesus declares that the pure in heart (katharos) will see God (Matthew 5:8). He also declares the disciples 'already clean' through the word (John 15:3).
Theologically, katharos is central to biblical holiness. Ritual impurity in the OT foreshadowed the deeper moral and spiritual purity that Christ accomplishes. The blood of Jesus cleanses (katharizo) from all sin (1 John 1:7) — making possible the heart purity that enables the vision of God.