endoskeleton vs skeleton

endoskeleton and skeleton both are nouns.

Word	Noun	Adjective	Verb	Adverb
endoskeleton	Yes	No	No	No
skeleton	Yes	No	No	No

As nouns, skeleton is a hypernym of endoskeleton; that is, skeleton is a word with a broader meaning than endoskeleton:

endoskeleton: the internal skeleton; bony and cartilaginous structure (especially of vertebrates)
skeleton: the hard structure (bones and cartilages) that provides a frame for the body of an animal

Other hypernyms of endoskeleton include frame, skeletal system, systema skeletale.

endoskeleton (noun)	skeleton (noun)
the internal skeleton; bony and cartilaginous structure (especially of vertebrates)	the internal supporting structure that gives an artifact its shape
	the hard structure (bones and cartilages) that provides a frame for the body of an animal
	a scandal that is kept secret
	something reduced to its minimal form