What is Accidental American?

Legal Definition
An accidental American is a citizen of a country other than the United States who may also be considered a U.S. citizen under U.S. nationality law but is not aware of having U.S. status, or has only become aware of it recently during adulthood. Accidental Americans' U.S. citizenship arises due to their parents' ties to the United States rather than their own choices: they may be born in their own country but to one U.S. citizen parent who emigrated from the United States, or they may be born in the U.S. to parents residing in the country temporarily for work or study and then return to their own country in their early childhood, with few if any memories of the United States. The term may also sometimes be applied to people who definitely are not U.S. citizens but have some other sort of connection with the country, for example green card holders who moved back to their country of origin and let their green cards expire without formally cancelling their U.S. immigration status, or non-U.S. citizens married to Americans abroad. Such tenuous connections to the United States began to become a more salient issue in the late 2000s due to Internal Revenue Service crackdowns which were ostensibly aimed at tax evaders hiding assets in secrecy jurisdictions but ended up having much broader effects on people with U.S. citizenship who resided in other countries, as well as their families.
-- Wikipedia