D-layer vs D region

D-layer and D region both are nouns.

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

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

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

D-layer (noun)	D region (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