midsection vs midriff

midsection and midriff both are nouns.

Word	Noun	Adjective	Verb	Adverb
midsection	Yes	No	No	No
midriff	Yes	No	No	No

As nouns, midsection and midriff are synonyms defined as:

midsection and midriff: the middle area of the human torso (usually in front)

Other synonyms of midsection include middle.

midsection (noun)	midriff (noun)
the middle area of the human torso (usually in front)	(anatomy) a muscular partition separating the abdominal and thoracic cavities; functions in respiration
	the middle area of the human torso (usually in front)