It occurs when a person's proficiency in their first language declines due to prolonged exposure to and use of a second language. This can happen for various reasons, including:
* Immigrating to a new country: When someone moves to a country where a different language is spoken, they may gradually lose fluency in their native tongue.
* Living in a bilingual environment: Spending a significant amount of time in an environment where a second language is dominant can lead to the decline of the first language.
* Lack of use: If a person doesn't regularly use their first language, they might lose proficiency.
It's important to note that language attrition doesn't always lead to a complete loss of the first language. It can also manifest as a decline in vocabulary, grammar, or fluency.