Illegal immigration refers to the migration of people into a country in violation of the immigration laws of that country, or the continued residence of people without the legal right to live in that country. Illegal immigration tends to be financially upward, from poorer to richer countries. Illegal residence in another country creates the risk of detention, deportation, and/or other sanctions.
There are campaigns discouraging the use of the term "illegal immigrant", generally based on the argument that the act of immigrating illegally does not make the people themselves illegal, but rather they are "people who have immigrated illegally".

Every nation, country, or colony often has rules and laws that control and regulate people who come in from other places.
Migration becomes illegal if people do not have the permission of the country or borders they are entering into. In recent times, illegal migration has been on the rise. Illegal migration is often fueled by pull factors.

People sneak into other countries by land or sea, and other organized groups help people to sneak into other countries to work illegally. This is known as human trafficking.

Many of the illegal migrants involved end up in very difficult and dangerous situations, as they do not have the proper documents to get a job.
The flow of illegal migrants is often from poorer countries to richer countries. The people involved often are not the poorest in their home countries. They tend to be people with a lot of information, knowledge, ambition, and motivation, which often fuels their desire to migrate for a better life.

These are people who live in a place without permission and the authorities have no record of them. It also includes people who visit a country for tourism or education or health purposes legally but do NOT go back.
This means that even though they went there legally, they are now illegal immigrants because they have out-stayed the time period they were given. In a similar way, immigrants who have expired documents, or who came in with fake documents all fall under undocumented immigrants.

Open immigration was initially encouraged because of the need to settle new territories. However, this changed throughout the 1800s with policies that banned or strictly limited Chinese and European immigrants. In the early 1900s, there was a quota system that the U.S. government used to determine the amount of each ethnic group that was allowed to immigrate while also promoting workers for needed jobs.