Adjective, Noun

A migrant is someone who moves temporarily to a new country while an immigrant is someone who will settle and stay permanently.

More specifically, migrant is an umbrella term, not defined under international law, reflecting the common lay understanding of a person who moves away from their place of usual residence, whether within a country or across an international border, temporarily or permanently, and for a variety of reasons. 

When someone enters the US without papers, referring to those people as undocumented immigrants, rather than illegal immigrants, promotes their humanity.

Source

Adapted from International Organization for Migration (IOM)