American pioneers are any of the people in American history who migrated west to join in settling

Settler

A settler is a person who has migrated to an area and established permanent residence there, often to colonize the area. Settlers are generally people who take up residence on land and cultivate it, as opposed to nomads...

 and developing new areas. The term especially refers to those who were going to settle any territory which had previously not been settled or developed by European or American society, although the territory may have been inhabited by or utilized by native peoples.

The pioneer

Settler

A settler is a person who has migrated to an area and established permanent residence there, often to colonize the area. Settlers are generally people who take up residence on land and cultivate it, as opposed to nomads...

 concept and ethos greatly predate the migration to parts of the United States now called Western, as many places now considered as East were also settled by pioneers from the coast.