That would be linguistic competency. This concept was first introduced
by Noam Chomsky and it is not to be confused with the concept of linguistic
performance. Linguistic performance
deals with how language is actually used in real communication while linguistic
competency is the system, the ideal system, which allows the speakers of the language to
see the differences between the correct sentences and the bad ones while also
allowing them to create and understand an infinite number of different
sentences in their language.
