manic depression vs manic depressive illness

manic depression and manic depressive illness both are nouns.

Word	Noun	Adjective	Verb	Adverb
manic depression	Yes	No	No	No
manic depressive illness	Yes	No	No	No

As nouns, manic depression and manic depressive illness are synonyms defined as:

manic depression and manic depressive illness: a mental disorder characterized by episodes of mania and depression

Other synonyms of manic depression include bipolar disorder, manic-depressive psychosis.

manic depression (noun)	manic depressive illness (noun)
a mental disorder characterized by episodes of mania and depression	a mental disorder characterized by episodes of mania and depression