Refers to a person who moves to another country or region to live. The direction (immigration vs. emigration) depends on context.
He started a new life as an immigrant to Canada.
19世紀には多くの移住民がアメリカに渡った。
In the 19th century, many migrants crossed over to America.
Compound of 移住 (いじゅう, 'migration') + 民 (みん, 'people').