a theory usually associated with Noam Chomsky which claims the existence of a human-innate universal grammar consisting of features that all natural human languages share, enabling children to acquire a language without being taught explicitly, but only having to set language-specific parameters during exposure to language input
The concept of Universal Grammar is based on a cognitive approach to the study of grammar. We can assume that native speakers have an internalized language within their minds. Such languages, also known as I-languages, are acquired at a very early stage of child development. Moreover, all children acquire native competence in their respective languages at about the same pace and receiving very little stimuli from adult speakers. A possible way to account for this ability to acquire a language is to posit the existence of a Universal Grammar, innate to the human cognitive apparatus. According to this hypothesis, children would have only to learn the particular aspects of their languages, such as the lexicon and some parameters (head-initial or head-final, pro-drop or non-pro-drop and so on).
Noam Chomsky, the founder of this field of research, defines UG as "the theory of human I-languages ...that identifies the I-languages that are humanly accessible under normal conditions"