lie down vs lie

lie down is not a noun while lie is a noun.

Word	Noun	Adjective	Verb	Adverb
lie down	No	No	Yes	No
lie	Yes	No	Yes	No

As verbs, lie down and lie are synonyms defined as:

lie down (noun)	lie (noun)
	a statement that deviates from or perverts the truth
	position or manner in which something is situated

lie down (verb)	lie (verb)
assume a reclining position	tell an untruth; pretend with intent to deceive
	be lying, be prostrate; be in a horizontal position
	assume a reclining position
	be located or situated somewhere; occupy a certain position
	originate (in)
	have a place in relation to something else
	be and remain in a particular state or condition