dialysis vs pheresis

dialysis and pheresis both are nouns.

Word	Noun	Adjective	Verb	Adverb
dialysis	Yes	No	No	No
pheresis	Yes	No	No	No

As nouns, pheresis is a hyponym of dialysis; that is, pheresis is a word with a more specific, narrower meaning than dialysis:

dialysis: separation of substances in solution by means of their unequal diffusion through semipermeable membranes
pheresis: a procedure in which blood is drawn and separated into its components by dialysis; some are retained and the rest are returned to the donor by transfusion

Other hyponyms of dialysis include apheresis, haemodialysis, hemodialysis.

dialysis (noun)	pheresis (noun)
separation of substances in solution by means of their unequal diffusion through semipermeable membranes	a procedure in which blood is drawn and separated into its components by dialysis; some are retained and the rest are returned to the donor by transfusion