Akathartos (from alpha-privative + kathairo, 'to cleanse') denotes whatever is ritually polluted, morally foul, or spiritually contaminated. Of its 32 New Testament occurrences, the overwhelming majority modify pneuma ('spirit') to form the phrase pneuma akatharton — 'unclean spirit' — the standard Synoptic designation for demonic beings (Mark 1:23-27; 5:2-13; Luke 4:36). This identification of demons as 'unclean' rather than merely 'evil' draws on the Levitical purity system, where akathartos translates Hebrew tame in the Septuagint for forbidden foods, bodily discharges, and contaminated objects (Lev 11-15). Paul extends the word to moral impurity among believers (2 Cor 6:17; Eph 5:5). Spanish 'inmundo,' French 'impur,' and German 'unrein' all preserve the fundamental purity-versus-defilement axis.