say-so vs declaration

say-so and declaration both are nouns.

Word	Noun	Adjective	Verb	Adverb
say-so	Yes	No	No	No
declaration	Yes	No	No	No

As nouns, declaration is a hypernym of say-so; that is, declaration is a word with a broader meaning than say-so:

say-so (noun)	declaration (noun)
the power or right to give orders or make decisions	a statement that is emphatic and explicit (spoken or written)
an authoritative declaration	a formal expression by a meeting; agreed to by a vote
one chap's arbitrary assertion	(law) unsworn statement that can be admitted in evidence in a legal transaction
	a formal public statement
	(contract bridge) the highest bid becomes the contract setting the number of tricks that the bidder must make
	a statement of taxable goods or of dutiable properties