Illegal immigration, also called clandestine migration or immigration and irregular migration or immigration, is the migration of people across national borders in a way that is contrary to the immigration laws of the destination country. In general, an undocumented immigrant (also called an illegal immigrant or unauthorized immigrantor other terms) is a person who enters and remains in a country without a valid visa or permit from that country, or who has overstayed the duration of a visa that has been granted, or whose visa has been cancelled. Some countries have millions of illegal immigrants. A refugee or asylum seeker is generally not considered an illegal immigrant, at least not until their refugee status has been determined.
Immigration, including illegal immigration, is overwhelmingly upward, from a poorer to a richer country. However, it is also noted that illegal immigrants tend not to be the poorest within their populations. When potential immigrants believe that the chances of successfully migrating are greater than the risks/costs, illegal immigration becomes an option. The benefits taken into account include not only expected improvements in income and living conditions, but also expectations in relation to potential future residential permits, where illegal immigrants are given a path to naturalization or citizenship. The costs may include restrictions on living as an illegal immigrant in the destination country, leaving family and ways of life behind, the experience of visible or verbal disdain by native-born residents in the host country, and the probability of being detained and resulting sanctions.