life buoy vs flotation device

life buoy and flotation device both are nouns.

Word	Noun	Adjective	Verb	Adverb
life buoy	Yes	No	No	No
flotation device	Yes	No	No	No

As nouns, flotation device is a hypernym of life buoy; that is, flotation device is a word with a broader meaning than life buoy:

life buoy: a life preserver in the form of a ring of buoyant material
flotation device: rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning

Other hypernyms of life buoy include life preserver, preserver.

life buoy (noun)	flotation device (noun)
a life preserver in the form of a ring of buoyant material	rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning