bathing vs sun bathing

bathing and sun bathing both are nouns.

Word	Noun	Adjective	Verb	Adverb
bathing	Yes	No	No	No
sun bathing	Yes	No	No	No

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

Other hyponyms of bathing include sea bathing.

bathing (noun)	sun bathing (noun)
immersing the body in water or sunshine	immersing the body in sunlight
the act of washing yourself (or another person)