viscera vs internal organ

viscera and internal organ both are nouns.

Word	Noun	Adjective	Verb	Adverb
viscera	Yes	No	No	No
internal organ	Yes	No	No	No

As nouns, internal organ is a hypernym of viscera; that is, internal organ is a word with a broader meaning than viscera:

viscera: internal organs collectively (especially those in the abdominal cavity)
internal organ: a main organ that is situated inside the body

Other hypernyms of viscera include viscus.

viscera (noun)	internal organ (noun)
internal organs collectively (especially those in the abdominal cavity)	a main organ that is situated inside the body