rain vs monsoon

rain and monsoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
rain	Yes	No	Yes	No
monsoon	Yes	No	No	No

As nouns, monsoon is a hyponym of rain; that is, monsoon is a word with a more specific, narrower meaning than rain:

rain (noun)	monsoon (noun)
anything happening rapidly or in quick successive	a seasonal wind in southern Asia; blows from the southwest (bringing rain) in summer and from the northeast in winter
water falling in drops from vapor condensed in the atmosphere	any wind that changes direction with the seasons
drops of fresh water that fall as precipitation from clouds	rainy season in southern Asia when the southwestern monsoon blows, bringing heavy rains

rain (verb)	monsoon (verb)
precipitate as rain