Final answer:
The term 'Pioneers' best defines the individuals who migrated west of the Mississippi in search of economic opportunities.
Explanation:
Pioneers best defines the thirteen million individuals that migrated west of the Mississippi in search of economic opportunities.
This migration can be compared to the movement of the 'Exodusters,' a group of Southern black migrants who sought homes in the Great Plains, hoping for political, legal, and economic freedom after Reconstruction.
In the post-Civil War era, Americans settled in the vast heart of the continent, moving beyond California and Oregon for new opportunities.
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