In neuropsychology, linguistics, and the philosophy of language, a natural language or ordinary language is any language that has evolved naturally in humans through use and repetition without conscious planning or premeditation. Natural languages can take different forms, such as speech or signing. They are distinguished from constructed and formal languages such as those used to program computers or to study logic.
Though the exact definition varies between scholars, natural language can broadly be defined in contrast to artificial or constructed languages (such as computer programming languages and international auxiliary languages) and to other communication systems in nature. Examples of such communication systems include bees' waggle dance and whale song, to which researchers have found or applied the linguistic cognates of dialect and even syntax. However, classification of animal communication systems as languages is controversial.
All language varieties of world languages are natural languages, although some varieties are subject to greater degrees of published prescriptivism or language regulation than others. Thus nonstandard dialects can be viewed as a wild type in comparison with standard languages. But even an official language with a regulating academy, such as Standard French with the French Academy, is classified as a natural language (for example, in the field of natural language processing), as its prescriptive points do not make it either constructed enough to be classified as a constructed language or controlled enough to be classified as a controlled natural language.
Controlled natural languages are subsets of natural languages whose grammars and dictionaries have been restricted in order to reduce or eliminate both ambiguity and complexity (for instance, by cutting down on languages used superlative or adverbial forms or irregular languages). The purpose behind the development and implementation of a controlled natural language typically is to aid non-native speakers of a natural language in understanding it, or to ease computer processing of a natural language. An example of a widely used controlled natural language is Simplified English, which was originally developed for aerospace industry manuals.
Constructed international auxiliary languages such as Esperanto and Interlingua (even those that have native speakers) are not generally considered natural languages. Natural languages have been used to communicate and have evolved in a natural way, whereas Esperanto was designed by L. L. Zamenhof selecting elements from natural languages, not grown from natural fluctuations in vocabulary and syntax. Some natural languages have become naturally "standardized" by children's natural tendency to correct for illogical grammatical structures in their parents' speech, which can be seen in the development of pidgin languages into creole languages (as explained by Steven Pinker in The Language Instinct), but this is not the case in many languages, including constructed languages such as Esperanto, where strict rules are in place as an attempt to consciously remove such irregularities. The possible exception to this are true native speakers of such languages. More substantive basis for this designation is that the vocabulary, grammar, and orthography of Interlingua are natural; they have been standardized and presented by a linguistic research body, but they predated it and are not themselves considered a product of human invention. Most experts, however, consider Interlingua to be naturalistic rather than natural. Latino sine flexione, a second naturalistic auxiliary language, is also naturalistic in content but is no longer widely spoken.