stop vs flag down

stop is a noun but flag down is not a noun.

Word	Noun	Adjective	Verb	Adverb
stop	Yes	No	Yes	No
flag down	No	No	Yes	No

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

Other hyponyms of stop include check, rein, rein in, arrest, halt, hold, bring up, cut, stall, stall, draw up, pull up, brake.

stop (noun)	flag down (noun)
the act of stopping something
a brief stay in the course of a journey
a restraint that checks the motion of something
an obstruction in a pipe or tube
a mechanical device in a camera that controls size of aperture of the lens
(music) a knob on an organ that is pulled to change the sound quality from the organ pipes
a consonant produced by stopping the flow of air at some point and suddenly releasing it
a punctuation mark (.) placed at the end of a declarative sentence to indicate a full stop or after abbreviations
the event of something ending
a spot where something halts or pauses
the state of inactivity following an interruption

stop (verb)	flag down (verb)
prevent completion	signal to stop
stop and wait, as if awaiting further instructions or developments
hold back, as of a danger or an enemy; check the expansion or influence of
seize on its way
render unsuitable for passage
come to a halt, stop moving
cause to end
interrupt a trip
stop from happening or developing
put an end to a state or an activity
have an end, in a temporal, spatial, or quantitative sense; either spatial or metaphorical