An illegal immigrant is someone who has entered a country without the government’s permission or has stayed beyond the expiration date of their visa. An undocumented immigrant is someone who does not have the legal right to be or remain in a country, but whose presence in the country has not yet been identified by the government.
The key difference between an illegal and undocumented immigrant is that an illegal immigrant has done something that is against the law (such as crossing a border without permission or overstaying a visa), while an undocumented immigrant has not necessarily broken any laws. Illegal immigrants can be subject to deportation, while undocumented immigrants may be able to stay in a country if they are able to prove their identities and gain legal status.