synovium vs membrane

synovium and membrane both are nouns.

Word	Noun	Adjective	Verb	Adverb
synovium	Yes	No	No	No
membrane	Yes	No	No	No

As nouns, membrane is a hypernym of synovium; that is, membrane is a word with a broader meaning than synovium:

synovium: a thin membrane in synovial (freely moving) joints that lines the joint capsule and secretes synovial fluid
membrane: a pliable sheet of tissue that covers or lines or connects the organs or cells of animals or plants

Other hypernyms of synovium include tissue layer.

synovium (noun)	membrane (noun)
a thin membrane in synovial (freely moving) joints that lines the joint capsule and secretes synovial fluid	a thin pliable sheet of material
	a pliable sheet of tissue that covers or lines or connects the organs or cells of animals or plants