white matter vs pathway

white matter and pathway both are nouns.

Word	Noun	Adjective	Verb	Adverb
white matter	Yes	No	No	No
pathway	Yes	No	No	No

As nouns, pathway is a hyponym of white matter; that is, pathway is a word with a more specific, narrower meaning than white matter:

white matter: whitish nervous tissue of the CNS consisting of neurons and their myelin sheaths
pathway: a bundle of myelinated nerve fibers following a path through the brain

Other hyponyms of white matter include nerve pathway, nerve tract, tract.

white matter (noun)	pathway (noun)
whitish nervous tissue of the CNS consisting of neurons and their myelin sheaths	a trodden path
	a bundle of myelinated nerve fibers following a path through the brain