Origionally used in reference to people deciding to change. Change resources, change behaviours, change a way of life, change their environment or their habitat. People on the move, going somewhere, progressing or escaping, changing.
It describes people moving from un-occupied land into non-occupied lands.
Emigration is people deciding to move from occupied land into alternative occupied lands.
Immigration is people being forced to moved from occupied land into occupied lands.
To be a Migrant you have to settle on non-occupied land or be moving through occupied lands.
Any person who stops in an occupied land is an Immigrant.