white matter vs tract

white matter and tract both are nouns.

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

As nouns, tract is a hyponym of white matter; that is, tract 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
tract: a bundle of myelinated nerve fibers following a path through the brain

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

white matter (noun)	tract (noun)
whitish nervous tissue of the CNS consisting of neurons and their myelin sheaths	a bundle of myelinated nerve fibers following a path through the brain
	a system of body parts that together serve some particular purpose
	a brief treatise on a subject of interest; published in the form of a booklet
	an extended area of land