swelling vs hydrops

swelling and hydrops both are nouns.

Word	Noun	Adjective	Verb	Adverb
swelling	Yes	No	No	No
hydrops	Yes	No	No	No

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

swelling: an abnormal protuberance or localized enlargement
hydrops: swelling from excessive accumulation of watery fluid in cells, tissues, or serous cavities

swelling (noun)	hydrops (noun)
the increase in volume of certain substances when they are heated (often accompanied by release of water)	swelling from excessive accumulation of watery fluid in cells, tissues, or serous cavities
something that bulges out or is protuberant or projects from its surroundings
an abnormal protuberance or localized enlargement