indisposition vs illness

indisposition and illness both are nouns.

Word	Noun	Adjective	Verb	Adverb
indisposition	Yes	No	No	No
illness	Yes	No	No	No

As nouns, illness is a hypernym of indisposition; that is, illness is a word with a broader meaning than indisposition:

indisposition: a slight illness
illness: impairment of normal physiological function affecting part or all of an organism

Other hypernyms of indisposition include malady, sickness, unwellness.

indisposition (noun)	illness (noun)
a certain degree of unwillingness	impairment of normal physiological function affecting part or all of an organism
a slight illness