Definitions for f layer
Random House Webster's College Dictionary
the highest radio-reflective region of the ionosphere, at an altitude of ab. 80 mi. (130 km).
Origin of F layer:
Appleton layer, F layer, F region(noun)
the highest region of the ionosphere (from 90 to 600 miles up) which contains the highest concentration of free electrons and is most useful for long-range radio transmission