The noun akatharsia (from a, not + katharos, clean) denotes moral and spiritual uncleanness — the state of being defiled, impure, or polluted. While it can refer to ceremonial uncleanness in a Jewish context, in the NT it predominantly describes the moral filthiness associated with sexual sin and the corruption of the flesh more broadly.
Paul lists akatharsia in two of his most important vice catalogs: Galatians 5:19 ('the acts of the flesh are: sexual immorality, impurity [akatharsia]…') and Colossians 3:5 ('put to death… sexual immorality, impurity [akatharsia]…'). The word describes not just individual acts but a state of being — the condition of a person given over to fleshly desires without the cleansing of the Spirit. Romans 1:24 records God's judgment on those who reject Him: He 'gave them over to akatharsia' — the terrifying abandonment of sinners to the very corruption they chose. The antidote is hagiasmos (G49, sanctification/holiness): 1 Thessalonians 4:7 states, 'God did not call us to be impure [akatharsia], but to live a holy life.' The believer's body is a temple of the Holy Spirit — akatharsia is a desecration of that temple.