Idiom is the syntactical, grammatical, or structural form peculiar to a language. Idiom is the realized structure of a language, as opposed to possible but unrealized structures that could have developed to serve the same semantic functions but did not. Language grammar and syntax is often inherently arbitrary and peculiar to a particular language or a group of related languages. For example, although in English it is idiomatic to say "cats are associated with agility", other forms could have developed, such as "cats associate toward agility" or "cats are associated of agility". Unidiomatic constructions sound wrong to fluent speakers, although they are often entirely comprehensible. For example, the title of the classic bookEnglish As She Is Spoke is easy to understand, but it deviates from English idiom in the gender of the pronoun and the inflection of the verb. Lexical gaps are another key example of idiom.
native speakers in an insulated monolingual-native environment are mostly not conscious of idiomaticness, because in general their minds never reach for, or hear, other possible structures. The main exception is when they hear the natural experimentation of children acquiring the language, when they may encounter, for example, overregularization. By this correlation, solecism to native-speaking monolingual minds often sounds childish. However, when adults study a foreign language, they become consciously aware of idiomaticness and the lack of it. For example, in English it is idiomatic to use an indefinite article when describing a person's occupation, but in Spanish and many other languages it is not, and a native speaker of English learning Spanish must encounter and accept that fact to become fluent. The countsense of the wordidiom, referring to a saying with a figurative meaning, is related to the present sense of the word by the arbitrariness and peculiarity aspects; the idiom "she is pulling my leg" is idiomatic because it belongs, by convention, to the language, whether or not anyone can identify the original logic by which it was coined, and regardless of whether it translates literally to any other language.
