unimaginable vs impossible

unimaginable is not a noun while impossible is a noun.

Word	Noun	Adjective	Verb	Adverb
unimaginable	No	Yes	No	No
impossible	Yes	Yes	No	No

As adjectives, unimaginable and impossible are synonyms defined as:

Other synonyms of unimaginable include inconceivable, out of the question.

unimaginable (noun)	impossible (noun)
	something that cannot be done

unimaginable (adjective)	impossible (adjective)
totally unlikely	not capable of occurring or being accomplished or dealt with
	(used of persons or their behavior) not acceptable or reasonable
	totally unlikely