Derived U.S. citizenship arises when

(1) you become a lawful permanent resident before you turn 18, and

(2) one of your parents becomes a U.S. citizen through naturalization before you turn 18.

Depending on your birth year, the law may require that both of your parents were naturalized before you turned 18. Like acquired citizenship, your birth year determines the requirements, which can be complicated. If you are in removal proceedings —but think you might be a U.S. citizen—hire a lawyer or accredited representative [2] to represent you. 

This article gives an overview of "derived U.S. citizenship" as a deportation defense. [3]