top brass vs organisation

top brass and organisation both are nouns.

Word	Noun	Adjective	Verb	Adverb
top brass	Yes	No	No	No
organisation	Yes	No	No	No

As nouns, organisation is a hypernym of top brass; that is, organisation is a word with a broader meaning than top brass:

top brass: the most important persons in a governing body
organisation: the persons (or committees or departments etc.) who make up a body for the purpose of administering something

Other hypernyms of top brass include administration, brass, establishment, governance, governing body, organization.

top brass (noun)	organisation (noun)
the most important persons in a governing body	the act of organizing a business or an activity related to a business
	the activity or result of distributing or disposing persons or things properly or methodically
	the act of forming or establishing something
	an ordered manner; orderliness by virtue of being methodical and well organized
	an organized structure for arranging or classifying
	a group of people who work together
	the persons (or committees or departments etc.) who make up a body for the purpose of administering something