D region vs D-layer

D region and D-layer both are nouns.

Word	Noun	Adjective	Verb	Adverb
D region	Yes	No	No	No
D-layer	Yes	No	No	No

As nouns, D region and D-layer are synonyms defined as:

D region and D-layer: the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves

D region (noun)	D-layer (noun)
the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves	the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves