Linguistic purism or linguistic protectionism is a concept with two common meanings: one with respect to foreign languages and the other with respect to the internal variants of a language (dialects). The first meaning is the historical trend of the users of a language desiring to conserve intact the language's lexical structure of word families, in opposition to foreign influence which are considered 'impure'. The second meaning is the prescriptive practice of determining and recognizing one linguistic variety (dialect) as being purer or of intrinsically higher quality than other related varieties.

The perceived or actual decline identified by the purists may take the form of a change of vocabulary, syncretism of grammatical elements, or loanwords.^{[citation needed]} The unwanted similarity is often with a neighboring language the speakers of which are culturally or politically dominant.^{[citation needed]} The ideal may invoke logic, clarity, or the grammar of classic languages. It is often presented as a conservative measure, as a protection of a language from the encroachment of other languages or of the conservation of the national Volksgeist, but is often innovative in defining a new standard. It is sometimes part of governmental language policy that is enforced in various ways.

Historically, in the first meaning, linguistic purism was institutionalized in Italy through language academies (of which the 1572 Accademia della Crusca set as a model example in Europe), and their decisions often having the force of law.

Purism in Italy until the 19th century stemmed from the doctrine, developed in the 16th century mainly by Pietro Bembo and Lionardo Salviati and supported by dictionaries and grammars, that literary usage should imitate 14th-century Florentine. This view was contested by the Enlightenment, and the terms purismo and purista (first recorded 1758–9 but not used in print until 1838) were introduced to denote linguistic affectation or archaism. The 19th-century purism of Antonio Cesari and others was based both on veneration for the 14th century and on distaste for neologisms and foreign borrowings, especially from French. Purism in the 20th century resisted unassimilated borrowings from French and later from English and was at its height during the second half of the Fascist period. The neopurismo promoted by Bruno Migliorini from the late 1930s sought a compromise between the needs of Italian to evolve and to maintain its structures.

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 to use different writing systems or different spelling systems.

Examples of this include: