sit down vs rest

sit down is not a noun while rest is a noun.

Word	Noun	Adjective	Verb	Adverb
sit down	No	No	Yes	No
rest	Yes	No	Yes	No

As verbs, rest is a hyponym of sit down; that is, rest is a word with a more specific, narrower meaning than sit down:

sit down: be in a position in which one's upper body is largely upright and supported by one's backside
rest: sit, as on a branch

Other hyponyms of sit down include lounge, sprawl, perch, roost, crouch, hunker, hunker down, scrunch, scrunch up, squat.

sit down (noun)	rest (noun)
	freedom from activity (work or strain or responsibility)
	a support on which things can be put
	a musical notation indicating a silence of a specified duration
	something left after other parts have been taken away
	a state of inaction
	euphemisms for death (based on an analogy between lying in a bed and in a tomb)
	a pause for relaxation

sit down (verb)	rest (verb)
show to a seat; assign a seat for	be at rest
be in a position in which one's upper body is largely upright and supported by one's backside	stay the same; remain in a certain state
take a seat	take a short break from one's activities in order to relax
	give a rest to
	not move; be in a resting position
	sit, as on a branch
	rest on or as if on a pillow
	put something in a resting position, as for support or steadying
	be inactive, refrain from acting
	have a place in relation to something else
	be inherent or innate in