grey substance vs nerve tissue

grey substance and nerve tissue both are nouns.

Word	Noun	Adjective	Verb	Adverb
grey substance	Yes	No	No	No
nerve tissue	Yes	No	No	No

As nouns, nerve tissue is a hypernym of grey substance; that is, nerve tissue is a word with a broader meaning than grey substance:

grey substance: greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons
nerve tissue: tissue composed of neurons

Other hypernyms of grey substance include nervous tissue.

grey substance (noun)	nerve tissue (noun)
greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons	tissue composed of neurons