gray substance vs gray matter

gray substance and gray matter both are nouns.

Word	Noun	Adjective	Verb	Adverb
gray substance	Yes	No	No	No
gray matter	Yes	No	No	No

As nouns, gray substance and gray matter are synonyms defined as:

gray substance and gray matter: greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons

Other synonyms of gray substance include grey matter, grey substance, substantia grisea.

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