The "Great American Desert" was the largely treeless plains region lying between the Rocky Mountains and the Mississippi River. In the 19th century, the word "desert" referred to any uninhabited region or an area still in its natural condition. When I was in grade school in the early 1950s many of the standard geography textbooks still had maps similar to this one, showing the "Great American Desert."
Adapted from a map of the United States, the British provinces, Mexico, etc., showing California's gold region and western trails, by John M. Atwood. Published by J.H. Colton, New York, 1849. Courtesy Library of Congress, Geography and Map Division.