lounge vs sit down

lounge is a noun but sit down is not a noun.

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

As verbs, sit down is a hypernym of lounge; that is, sit down is a word with a broader meaning than lounge:

lounge: sit or recline comfortably
sit down: be in a position in which one's upper body is largely upright and supported by one's backside

Other hypernyms of lounge include sit.

lounge (noun)	sit down (noun)
a room (as in a hotel or airport) with seating where people can wait
an upholstered seat for more than one person

lounge (verb)	sit down (verb)
sit or recline comfortably	show to a seat; assign a seat for
be about a place without any apparent purpose	be in a position in which one's upper body is largely upright and supported by one's backside
	take a seat