F Layer Definition

The highest region of the ionosphere, extending at night from about 190 to 400 kilometers (120 to 250 miles) and during the day from about 145 to 400 kilometers (90 to 250 miles) above the earth.
American Heritage
The two dense layers of ions in the F region of the ionosphere that are used to reflect long-range radio signals: the F1 layer, at an altitude of c. 200 to 300 km (c. 125 to 185 miles), exists during the day, and the permanent F2 layer, or Appleton layer, is at c. 225 to 400 km (c. 140 to 250 miles)
Webster's New World