The phrase "illegal migrants" refers to people who enter or stay in a country without permission from the government or proper documentation. They are called "illegal" because their presence in the country is against the law. Full definition
Employers shouldn't be taking advantage of illegal migrants looking for work here — and that's what happens when people look the other way. (nystateofpolitics.com)
In her own words, the aim of the act was: «to create a really hostile environment for illegal migrants». (corporatewatch.org)
Canadian officials have also asked the Chinese authorities for help to stem the tide of illegal migrants. (asiapacific.ca)