feather bed vs mattress

feather bed and mattress both are nouns.

Word	Noun	Adjective	Verb	Adverb
feather bed	Yes	No	No	No
mattress	Yes	No	No	No

As nouns, mattress is a hypernym of feather bed; that is, mattress is a word with a broader meaning than feather bed:

feather bed: a mattress stuffed with feathers
mattress: a large thick pad filled with resilient material and often incorporating coiled springs, used as a bed or part of a bed

feather bed (noun)	mattress (noun)
a mattress stuffed with feathers	a large thick pad filled with resilient material and often incorporating coiled springs, used as a bed or part of a bed