brainstem vs brain stem

brainstem and brain stem both are nouns.

Word	Noun	Adjective	Verb	Adverb
brainstem	Yes	No	No	No
brain stem	Yes	No	No	No

As nouns, brainstem and brain stem are synonyms defined as:

brainstem and brain stem: the part of the brain continuous with the spinal cord and comprising the medulla oblongata and pons and midbrain and parts of the hypothalamus

Other synonyms of brainstem include brain-stem.

brainstem (noun)	brain stem (noun)
the part of the brain continuous with the spinal cord and comprising the medulla oblongata and pons and midbrain and parts of the hypothalamus	the part of the brain continuous with the spinal cord and comprising the medulla oblongata and pons and midbrain and parts of the hypothalamus