central nervous system vs encephalon

central nervous system and encephalon both are nouns.

Word	Noun	Adjective	Verb	Adverb
central nervous system	Yes	No	No	No
encephalon	Yes	No	No	No

central nervous system (noun)	encephalon (noun)
the portion of the vertebrate nervous system consisting of the brain and spinal cord	that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord