doom vs doomsday

doom and doomsday both are nouns.

Word	Noun	Adjective	Verb	Adverb
doom	Yes	No	Yes	No
doomsday	Yes	No	No	No

As nouns, doom and doomsday are synonyms defined as:

Other synonyms of doom include day of reckoning, end of the world.

doom (noun)	doomsday (noun)
an unpleasant or disastrous destiny	an unpleasant or disastrous destiny
	(New Testament) day at the end of time following Armageddon when God will decree the fates of all individual humans according to the good and evil of their earthly lives

doom (verb)	doomsday (verb)
pronounce a sentence on (somebody) in a court of law
decree or designate beforehand
make certain of the failure or destruction of