A plain or a flatland was an area of land with little or no change in elevation.
Flatlands stretched beyond the Pointed Peaks on Delta Vega. (TOS: "Where No Man Has Gone Before")
Vulcan's Anvil was known for its' "blistering plains." (DS9: "The Homecoming")
In 2267, according to the Greek god Apollo, Humans had left the plains and valleys of Earth and ventured into space. (TOS: "Who Mourns for Adonais?")