universal grammar (UG) (noun): a theory in linguistics usually credited to Noam Chomsky that suggests that the ability to learn grammar is built into the human brain from birth regardless of language
In the 1960s, linguists became interested in a new theory about grammar, or the laws of language. The theory was popularized by an American linguist named Noam Chomsky who often focused on the effortless language learning of young children.
Chomsky didn’t believe that exposure to a language was enough for a young child to become efficient at understanding and producing a language. He believed that humans are born with an innate ability to learn languages. According to Chomsky’s theory, the basic structures of language are already encoded in the human brain at birth.
This “universal grammar theory” suggests that every language has some of the same laws. For example, every language has a way to ask a question or make something negative. In addition, every language has a way to identify gender or show that something happened in the past or present.
If the basic grammar laws are the same for all languages, a child needs only to follow the particular set of rules that his peers follow in order to understand and produce their native language. In other words, his environment determines which language he will use, but he is born with the tools to learn any language effectively.