What is the name for a pidgin language that has become mother tongue?

A pidgin language that has become a mother tongue is called a creole.

Here's the breakdown:

* Pidgin: A simplified language that develops for communication between speakers of different languages. It has a limited vocabulary and grammar, and is typically used in specific situations, like trade or business.

* Creole: A language that develops from a pidgin when it is acquired by children as their first language. It expands its vocabulary and grammar, becoming more complex and expressive.

So, while a pidgin language is a simplified form of communication, a creole language is a fully-fledged language in its own right, having evolved from a pidgin.