shout out vs halloo

shout out is not a noun while halloo is a noun.

Word	Noun	Adjective	Verb	Adverb
shout out	No	No	Yes	No
halloo	Yes	No	Yes	No

As verbs, halloo is a hyponym of shout out; that is, halloo is a word with a more specific, narrower meaning than shout out:

Other hyponyms of shout out include hollo, hurrah, whoop, pipe, pipe up, shriek, shrill, howl, roar, ululate, wail, yaup, yawl, screak, screech, skreak, skreigh, squawk.

shout out (noun)	halloo (noun)
	a shout to attract attention

shout out (verb)	halloo (verb)
utter a sudden loud cry	shout `halloo', as when greeting someone or attracting attention
utter in a very loud voice	urge on with shouts