unclean: defiled and unfit to come near what is holy, the opposite of pure.
TranslatabilityPartialCertaintySettled
This word describes anything that is impure and therefore out of place before a holy God. It draws on Israel's old distinction between clean and unclean, where certain foods, conditions, and contacts made a person unfit to approach the sanctuary until they were cleansed.
In the Gospels the word takes on a striking use: it is the standard label for the evil spirits Jesus casts out. To call a spirit unclean is to say it is the exact contrary of God's own Spirit, defiling rather than holy, and that it has no business in a person or in God's presence. This is far and away the most common use in the New Testament.
The same idea reaches into moral and spiritual life. A person can be unclean not by touching the wrong thing but by living an impure life, especially through sexual sin. And in the good news that the Gospels announce, the old food and contact rules are set aside: God tells Peter to call no person unclean, so that the barrier between Jew and Gentile falls. Across all these uses the core holds steady: unclean means defiled, unfit for the holy.
Nuances
1aof an evil spirit: the fixed phrase "unclean spirit," meaning a demon, the opposite of God's Holy Spirit.
1britually unclean: a food, animal, or person that the purity laws mark as unfit to approach the holy, a category the New Testament shows God setting aside.
the biblical word ties impurity to what is fit or unfit for God, not merely to a mixture or contaminationEph 5:5
Both
both describe a state of defilement, especially moralRev 17:4
Only impure
"impure" in English often means adulterated or tainted in a neutral, physical sense
“evil (of a spirit)”
Comparison data is not available for this English gloss yet.
How akathartos differs from similar Greek words
κοινόςcommon: also rendered unclean, but it stresses being ordinary or shared rather than set apart, whereas this word stresses active defilement; the two stand side by side in Peter's vision.
By far its most common use is for evil spirits, the unclean spirit that seizes and torments a person, so it names an impurity that is active and hostile, not just a ritual stateMrk 1:23Mrk 5:8Luk 8:29
It brands a person or thing as impure in itself, the opposite of holy, whether from disease, sin, or contact with what defiles2Co 6:17Eph 5:5Rev 17:4
Both
In Peter's vision the two are paired as near-equivalents for what the law forbids a Jew to eat or touch, both marking food and people as ritually off-limitsAct 10:14Act 10:28
Its plain, everyday meaning is shared or held in common, which carries no stain at all, as when the first believers owned everything togetherAct 2:44Act 4:32Jud 1:3
When it does mean defiled it points to what is ordinary and unconsecrated rather than actively impure, food or hands treated as off-limits because they were not kept set apartMrk 7:2Mrk 7:5Rom 14:14
ἀκαθαρσίαuncleanness: the noun for the state or condition of impurity itself, where this word is the adjective describing what is in that state.
ἅγιοςholy: its direct opposite; where this word means defiled and unfit for God, holy means set apart and belonging to God.
akathartos in the other biblical languages
טָמֵאHebrew“unclean”the main Hebrew word for unclean in the purity laws, the background this Greek word draws on and often translates.
Most often names a demon: an "unclean spirit" that possesses a person and is driven out. This is the dominant New Testament use and has no counterpart in the Hebrew word, which never labels a spirit or evil being.Mrk 1:23Mrk 5:8Mat 10:1
Both
Both name ritual unfitness that bars a person or thing from the holy and forces separation: do not touch what is unclean. The Greek word is the standard rendering of the Hebrew, and Paul quotes this very purity language.Lev 5:22Co 6:17
Both stretch from ritual defilement to moral and spiritual defilement, so that sin, idolatry, and impure desire are spoken of as uncleanness that shuts a person out from God.Isa 6:5Eph 5:5
Anchors a fixed legal classification of animals as clean or unclean for food: each creature is permanently labeled, and the list itself is the law. The Greek word is never used to set up a standing catalogue of permitted and forbidden animals this way.Lev 11:4Lev 11:47Deu 14:7
Marks a person made physically unclean by skin disease, bodily discharge, or contact with a corpse, with the priest diagnosing it and the person quarantined until restored. The Greek word carries none of this detailed bodily and priestly machinery.Lev 13:45Lev 15:25Num 19:13
Its most common use in the New Testament is of a hostile spirit that seizes and torments a person, an "unclean spirit" that Jesus and his followers drive out.Mat 12:43Mrk 1:23Mrk 5:2Luk 4:33Act 5:16
It labels a state that can be lifted or declared void: Peter is told to call no person "unclean", and the child of a believing parent is counted holy rather than unclean.Act 10:281Co 7:14
It stretches from ritual uncleanness to moral filth, naming a corrupt way of living that shuts a person out of God's kingdom.Eph 5:5Rev 17:4
Both
Both belong to the food laws, marking what must not be eaten. When Peter protests that he has never eaten anything "common or unclean", the Greek word is covering exactly the creatures the Hebrew word rules out.Lev 11:10Lev 11:41Act 10:14Act 11:8
Both stand behind a command to keep clear of what defiles, so that a person stays fit to come near God: touching one bars an Israelite from the sacrificial meal, and God's people are told to touch no unclean thing.Lev 7:212Co 6:17
It names the repulsive creature itself rather than describing a condition, and it belongs to one fixed list: the water creatures, birds, and small swarming animals Israel may not eat.Lev 11:10Lev 11:12Lev 11:13Lev 11:20Lev 11:41
It demands active loathing, not just avoidance: Israel is told to hold these creatures and even their carcasses in disgust.Lev 11:11Lev 11:23Lev 11:42
It turns up where these same creatures become objects of false worship: their shapes carved around the temple wall, and their flesh eaten in a pagan rite.Ezk 8:10Isa 66:17
What scholars say about akathartos
Standard lexica (BDAG, LSJ) treat ἀκάθαρτος as a single adjective, the alpha-privative negation of καθαρός, spanning cultic, demonological, and moral impurity under one semantic field of impurity opposed to the holy. BDAG distinguishes three usages without positing homonymy: (1) ceremonial/levitical uncleanness of foods, persons, and things (Acts 10:14, 28; 11:8; 2 Cor 6:17, echoing Isa 52:11); (2) the fixed collocation πνεῦμα ἀκάθαρτον for demonic spirits, dominant in the Synoptics and Acts and paralleled in the LXX (Zech 13:2); and (3) moral impurity, especially sexual (Eph 5:5;Rev 17:4). TDNT (Hauck) and NIDNTT trace the demonological sense to the Jewish opposition between the unclean spirit and God's Holy Spirit. The referent is well established; scholarly discussion concerns the relative weight and background of the demonological usage, not the word's meaning.
Sources
BDAG, s.v. ἀκάθαρτοςlexicon
LSJ, s.v. ἀκάθαρτοςlexicon
F. Hauck, "ἀκάθαρτος, ἀκαθαρσία," TDNT 3:427-429dictionary
H. Balz and G. Schneider, EDNT 1:39, s.v. ἀκάθαρτοςdictionary