drain vs brain drain

drain and brain drain both are nouns.

Word	Noun	Adjective	Verb	Adverb
drain	Yes	No	Yes	No
brain drain	Yes	No	No	No

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

Other hyponyms of drain include drawing, drawing off.

drain (noun)	brain drain (noun)
emptying something accomplished by allowing liquid to run out of it	depletion or loss of intellectual and technical personnel
a gradual depletion of energy or resources
a pipe through which liquid is carried away
tube inserted into a body cavity (as during surgery) to remove unwanted material

drain (verb)	brain drain (verb)
empty of liquid; drain the liquid from
make weak
deplete of resources
flow off gradually