Linguistic purism or linguistic protectionism is the practice of defining or recognizing one variety of a language as being purer or of intrinsically higher quality than other varieties. Linguistic purism was institutionalized through language academies (of which the 1572 Accademia della Crusca set a model example in Europe), and their decisions often have the force of law. 
The perceived or actual decline identified by the purists may take the form of change of vocabulary, syncretism of grammatical elements, or loanwords. Linguistic purism is a form of prescriptive linguistics.  The unwanted similarity is often with a neighboring language whose speakers are culturally or politically dominant. The abstract ideal may invoke logic, clarity, or the grammar of "classic" languages. It is often presented as conservative, as a "protection" of a language from the "aggression" of other languages or of "conservation" of the national Volksgeist, but is often innovative in defining a new standard. It is sometimes part of governmental language policy which is enforced in various ways.
In one common case, two closely related languages or language varieties are in direct competition, one weaker, the other stronger. Speakers of the stronger language may characterize the weaker language as a " dialect " of the strong language, with the implication that it has no independent existence. In response, defenders of the other language will go to great lengths to prove that their language is equally autonomous.
In this context, Yiddish and Dutch have in the past sometimes been considered dialects of German. In the case of Low German, spoken in eastern Netherlands and northern Germany, the debate is still current, as it could be considered a dialect of Dutch or German or a language of its own. An example of a related language that has only recently attained the status of an official national language is Luxembourgish. Since linguistic science offers no scholarly definition of a dialect, and linguists regard the distinction with scepticism – see A language is a dialect with an army and navy – the argument is really about subjective questions of identity politics, and at times it can invoke extreme emotions from the participants.
Closely related languages often tend to mix. One way of preventing this is using different writing systems or different spelling systems.
Examples of this include:
- The Moldovan and Romanian languages are virtually identical in all respects except that Moldovan used Cyrillic script – which is still in use in Transnistria – and Romanian uses Latin script.
- Yiddish is very close to German, but uses the Hebrew alphabet instead of the Latin alphabet, and so keeps its separateness. This results in the situation where, for example, an Israeli could read a Yiddish text out loud to a German who could not read Hebrew, and the German would understand it, while the Israeli would not.
- Hindi and Urdu, traditionally kept separated by using Devanagari and Arabic script, respectively. This is a well-known example often cited in linguistic texts; however, in recent decades, it has been observed that the languages are tending to drift much further apart, due to the Sanskritization and Anglicization of Hindi and the Arabization and Persianization of Urdu.
- The Serbian and Croatian languages differ mainly in using Cyrillic and Latin scripts, respectively. Both of them exhibit high degree of mutual intelligibility as both standard Serbian and Croatian are based on essentially the same dialect (stylised form of Neoštokavian).
Various scholars have devised classifications of purism. These classifications take different criteria as their starting point and are therefore partly independent of each other.
Based on the approach
- Archaizing purism : This occurs when a speech community tries to resuscitate the language of a perceived or actual golden age of literature. Examples: Arabic, Tanittamil Iyakkam in Tamil language, Icelandic (see also: Linguistic purism in Icelandic), Katharevousa Greek, Sanskrit, Latin (puristic obsession with classical forms during the Renaissance). See also Language revival.
- Ethnographic purism : This form is based on an idealization of the countryside, folk stories and dialects. Examples: Nynorsk (New Norwegian), some versions of Demotic Greek.
- Elitist purism : Associated with a highly formal variety linked to an elite, for example the language spoken at the court.
- Reformist purism : The main feature here is to break the bonds with the past. An example of this is the removal of Persian and Arabic words during Turkish language reform under Atatürk in order to break with the Ottoman Turkish language influenced by Arabic and Persian. Other examples are the purist efforts in languages like Hausa, Swahili and Hindi to break with the colonial past. In addition, language policies may seek to decrease similarities between mutually intelligible languages for ethno-political reasons, as has been the case with Dano-Norwegian, Hindustani ( Hindi and Urdu) and Malaysian / Indonesian.
- Patriotic purism : involves the elimination or exclusion of foreign elements. Examples include High Norwegian, Korean and Anglish. Many English writers of the 19th and 20th centuries extolled the virtues of "strong" Anglo-Saxon words such as foreword , which was coined to replace the "weak" Romance word preface . France, Germany and Greece are known for their preference for coining words over borrowing foreign words (but this does not always work well, since many French words or expressions coined to avoid the use of a loanword (especially an English one) have not been adopted in common language, or are used only marginally).
This classification of puristic orientations made by George Thomas represents ideal forms. In practice, though, these orientations are often combined.
Based on the goals
- Democratic purism : Aims at safeguarding the intelligibility of (modern) concepts for a larger group of language users through enforcing their expression by the means of common, every-day words or expressions (for example, “back[ing] up” instead of “sustain[ment]”)
- Unificatory purism : Aims at better uniting the overall user group of a language by reducing certain regional or professional linguistic peculiarities which could separate varying aspects of life, or even obstruct interconnectivity, between individuals or sub-groups of different regional provenience or professional background.
- Defensive purism : Aims at defending a language from external threats. Mostly, these are to be understood as influx of foreign ideas which a given language group (or its political system) disdains or has overthrown, or influx of foreign words or expressions which tend to substitute innate vocabulary, thus diminishing and/or endangering supra-regional or inter-generational intelligibility within a language area or between its present speakers and the literary remnants of their venerated ancestors, i. e., some kind of “classical” heritage (as e. g. Shakespeare's usage is already no more widely understood amongst many of today's English speakers).
- Prestige purism : Aims at varying prestige functions.
- Delimiting purism : Aims at establishing some kind of separating functions.
Based on the intensity
- Marginal purism : In this pattern purism never becomes at any stage a value-feature of the speech community. On the contrary, there is a certain openness to all sources of enrichment, at the same time characterized by a lack among the language elite of intellectual digestion of foreign influxes, or by a lack of such an elite as a whole. Examples: English, Russian, Polish, Japanese.
- Moderate, discontinuous purism : In this pattern, a moderate attitude is discernible over a long period of time. Examples: Spanish, Portuguese, French and Italian.
- Trimming purism : A reactive correction to a potentially dangerous trend in the development of a standard language. Examples: Danish, Swedish, Dutch, Slovak.
- Evolutionary purism : In this pattern purism is seen early in the development of a written language. There are no radical changes or orientation. During the standardisation process, the purism gains momentum after which it slows down. Examples: Hungarian, Finnish, Estonian, Hebrew, Croatian and Slovene.
- Oscillatory purism : Involves repeated swings between intense purism and a more inclusive attitude. Examples: German, Czech and Yiddish.
- Stable, consistent purism : In this pattern no interruption or fluctuation in intensity is seen. Purism is a constant value-feature of the speech community. Examples: Arabic, Tamil and Icelandic.
- Revolutionary purism : An abrupt and violent change from the previously mentioned patterns to another. Examples: Turkish.
Based on linguistic level
- Lexical purism : directed at the lexicon, first of all against direct lexical loans, often combined with the development of loan translations (such as in Norwegian: hand out > støtteark and snowboard > snøbrett or Arabic tilifūn > hātif and kumbyūtir > ḥāsūb .
- Orthographic purism : directed against foreign orthographic elements (such as in Norwegian: genre > sjanger , in Spanish: football > fútbol ). Note that there is also reverse orthographic purism. Some Spanish speakers prefer the English spelling "blue jean"  and object to the spelling bluyín . 
- Morphological purism : directed against foreign inflection and declension (such as the resistance to plural -s in noun endings in Scandinavian languages).
- Syntactic purism : directed at syntactic features from other languages (such as the stylistic resistance in Nynorsk against some passive constructions and some constructions with the genitive).
- Phonetic purism : directed at foreign phonemes and phonematical combinations (such as gánster  or champú  in Spanish). Note that there is a reverse phonetic purism, which insists in the original pronunciation, such as pronouncing gá ng ster and sh ampú in Spanish.
- Regressive purism : The eradication of very old loan-words. It is one of the main features of ultrapurism.
- Ultrapurism : The extreme upper limit of purism. In this pattern, everything expressed by human speech can become a target for puristic intervention, even geographical names, proper names, and names of physical elements, chemicals, etc. (It should be noted, however, that the attitude – in itself "puristic" and associated with increased education and foreign language competency – opposed to the translation or adaptation of toponyms, or even personal names, is historically quite recent, as names are not considered fixed or unchanging in most cultures; and there are many exceptions even in English, especially the names of historical personages, Native Americans, and even contemporary royalty. Historically, names were part of the lexicon of a language just as every other word, and it was common to have different names associated with different language communities. See exonym and endonym. Also, the names of physical elements and chemicals, or astronomical objects, do vary between languages, sometimes quite radically. The longer established the tradition of a name or term, the more likely are strong differences.) The only two recorded examples of this are High Icelandic (Háfrónska), and the usage of the German renaissance humanist Johann Georg Turmair who even translated the name of the ancient Roman general Fabius Cunctator into Zauderer Bohnenmaier (i. e. literally “Laggard Bean-Mayor”). While not ultra-purism per se, phono-semantic matching is commonly used in a number of languages, notably for translating proper names into Chinese.
Linguistic purism by language