British West Africa vs dominion

British West Africa and dominion both are nouns.

Word	Noun	Adjective	Verb	Adverb
British West Africa	Yes	No	No	No
dominion	Yes	No	No	No

As nouns, dominion is a hypernym of British West Africa; that is, dominion is a word with a broader meaning than British West Africa:

British West Africa: the former British territories of western Africa, including Nigeria, Cameroon, Gambia, Togo, Sierra Leone, and the Gold Coast
dominion: a region marked off for administrative or other purposes

Other hypernyms of British West Africa include district, territorial dominion, territory.

British West Africa (noun)	dominion (noun)
the former British territories of western Africa, including Nigeria, Cameroon, Gambia, Togo, Sierra Leone, and the Gold Coast	a region marked off for administrative or other purposes
	dominance or power through legal authority