Plains are level areas of land. Nephi saw “the plains of the earth” broken up amid earthquakes, while mountains tumbled into pieces and cities were sunk, burned, or cast down by quaking (1 Nephi 12:4).
Moroni’s forces sent embassies to Jacob, leader of the Lamanite army protecting the city of Mulek, asking him to meet them “upon the plains between the two cities”. Jacob, identified as a Zoramite, would not come out with his army upon the plains (Alma 52:20).