An undocumented person is someone who is in a country without permission from that country’s government.
Each country makes immigration laws in order to keep record of who is visiting or living in their country. Everyone requires some type of permission – in the form of a visa – to be in another country, and all visas specify the time duration of the permission.
People are considered undocumented for one of two reasons: Some enter a country with permission and instead of leaving when their permission expires, they stay. Other people will enter a country without ever receiving permission.
When police, or other law enforcement officers, find an undocumented person, they often arrest them and deport that person back to their country of citizenship.