Concept of L1 (First Language)

L1 is the first language an individual acquires in her/ his life. It is the language this individual feels more comfortable with. It is also known as mother tongue. L1 is commonly acquired during childhood, but it does not mean that it is the first language  a child is exposed to. For example, a child can be born in the US and be in contract with English language for about a month but then he/she goes live in another country that another language is spoken in, and the child's first language, or mother tongue, will be the language the child was raised in contact. It is the first language an individual internalizes and uses for communication within ethnic, family contexts.

( https://global-lt.com/wp-content/uploads/2016/08/myth.jpg?19fc30&19fc30 )

FURTHER READING:

First language. Wikipedia. https://en.m.wikipedia.org/wiki/First_language