In English translations of the Bible, unclean spirit is a common rendering of Greek pneuma akatharton (πνεῦμα ἀκάθαρτον; plural pneumata akatharta (πνεύματα ἀκάθαρτα)), which in its single occurrence in the Septuagint translates Hebrew ruaḥ tum'ah (רוּחַ טוּמְאָה).

The Greek term appears 21 times in the New Testament in the context of demonic possession. It is also translated into English as spirit of impurity or more loosely as "evil spirit." The Latin equivalent is spiritus immundus.

The association of physical and spiritual cleanliness is, if not universal, widespread and continues into the 21st century: "To be virtuous is to be physically clean and free from the impurity that is sin," notes an article in Scientific American published 10 March 2009. Some scholarship seeks to differentiate between "unclean spirit" and "evil spirit" (pneuma ponêron) or "demon" (daimonion).

In the Christian scriptures, the word pneuma (plural pneumata) is used variously for the human soul, angelic or demonic spirits, and the Holy Spirit, depending on context or with a grammatical modifier. New Testament usage of the words pneuma and daimonion in relation to demons follows that of later Judaism; the two words are to be distinguished from daimon, which appears only once (at Matthew 8:31) and in classical antiquity has a neutral meaning of "spirit" or "god, demigod." For those who practiced the traditional religions of antiquity, possession by a pneuma could be a desired state of visionary trance.

In the New Testament, the Greek modifier akatharton, although sometimes translated in context as "evil," means more precisely "impure, not purified," and reflects a concern for ritual purification shared with or derived from Judaism, though reinterpreted. In early Christianity, the catechumen was routinely prepared for baptism by exorcism even when demonic possession was not suspected; in the case of adult converts, the "unclean spirits" to be driven away might be identified with the gods of other religions.

The practice of insufflation and exsufflation, or the use of released breath in ritual, depends on conceptualizing a spiritual entity as air in motion, "invisible yet active": both Greek pneuma and Latin spiritus had an original meaning of "breath, mobile air."

References to a "spirit of impurity" or an "evil spirit" (ruaḥ tum'ah) are found in the Hebrew Bible, in Rabbinic literature, and in Pseudepigrapha. It can be difficult to distinguish between a demon and an unclean or evil spirit in Judaic theology or contemporary scholarship; both entities like to inhabit wild or desolate places. Commonly the unclean spirit refers to Dybbuks, spirits of deceased persons who were not laid to rest and thus became demons. Other demonic entities are shedim, which appears only twice in the Tanakh; originally a loan-word from Akkadian for a protective, benevolent spirit (sedu), but from Jewish perspective were foreign gods and according to established Jewish lore own beings, created by Jahwe.

The se’irim or śa‘ir are goat-demons or "hairy demons" (sometimes translated as "satyrs") associated with other harmful supernatural beings and with ruins, i.e., human structures that threaten to revert to the wild. The demonic figure Azazel, depicted with goat-like features and in one instance as an unclean bird, is consigned to desert places as impure. The Babylonian Talmud says that a person who wanted to attract an impure spirit might fast and spend the night in a cemetery; in the traditional religions of the Near East and Europe, one ritual mode for seeking a divinely inspired revelation or prophecy required incubation at the tomb of an ancestor or hero. A cemetery, already a locus of "unclean" spirits or a multiplicity of gods, was considered an appropriate dump when biblical leaders destroy sacred objects of other religions or statues representing the gods. To pneuma to akatharton appears in the Septuagint at Zechariah 13:2, where pseudoprophetai ("false prophets") speak in the name of Yahweh but are possessed by an unclean spirit. This occurrence of "unclean spirit" is unique in the Tanakh; the Hebrew is rûah hattum’â.