Universal grammar (UG) is a theory in linguistics that suggests that the ability to learn grammar is built into the human brain. It is usually credited to Noam Chomsky, who proposed that the human brain contains an innate mental grammar that helps humans acquire language. Chomsky theorized that the brain contains a mechanism he referred to as a language acquisition device (LAD), which is separate from other faculties of cognitive activity. According to Chomsky, children acquire their mental grammar spontaneously and without formal training, and children of the same speech community reliably learn the same grammar.

The basic postulate of UG is that there are innate constraints on what the grammar of a possible human language can be. Universal grammar consists of a set of atomic grammatical categories and relations that are the building blocks of the particular grammars of all human languages, over which syntactic structures and constraints on those structures are defined. The theory suggests that every language has some of the same laws, such as a way to ask a question or make something negative, and a way to identify gender or show that something happened in the past or present.

However, there is little agreement on what exactly is in UG, and the empirical evidence for it is very weak. The concept of UG has been criticized and controversial among scholars in the scientific community. Some argue that there are no linguistic universals, and that UG is refuted by abundant variation at all levels.

In summary, Universal Grammar is a theoretical concept that suggests that the ability to learn grammar is built into the human brain, and that every language has some of the same laws. However, there is little agreement on what exactly is in UG, and the empirical evidence for it is very weak.