coat vs greatcoat

coat and greatcoat both are nouns.

Word	Noun	Adjective	Verb	Adverb
coat	Yes	No	Yes	No
greatcoat	Yes	No	No	No

As nouns, greatcoat is a hyponym of coat; that is, greatcoat is a word with a more specific, narrower meaning than coat:

coat: an outer garment that has sleeves and covers the body from shoulder down; worn outdoors
greatcoat: a heavy coat worn over clothes in winter

coat (noun)	greatcoat (noun)
growth of hair or wool or fur covering the body of an animal	a heavy coat worn over clothes in winter
an outer garment that has sleeves and covers the body from shoulder down; worn outdoors
a thin layer covering something

coat (verb)	greatcoat (verb)
cover or provide with a coat
put a coat on; cover the surface of; furnish with a surface
form a coat over