Green Card
In short
A green card is an official identity document that proves a person has lawful permanent residency in the United States.
In plain words
It allows non-citizens to live and work in the US indefinitely. Holders are called lawful permanent residents and can eventually apply for citizenship.
A simple example
A person moves from the UK to New York for a long-term job and receives this card to prove they can stay legally.
Why it matters
It provides essential legal rights, such as the ability to work almost anywhere and access to social services. It is a major step towards becoming a US citizen.
Easy to mix up
Do not confuse it with a US passport, which is only for US citizens.