mattress vs feather bed

mattress and feather bed both are nouns.

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

As nouns, feather bed is a hyponym of mattress; that is, feather bed is a word with a more specific, narrower meaning than mattress:

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: a mattress stuffed with feathers

Other hyponyms of mattress include air mattress, featherbed, futon, paillasse, palliasse, pallet, spring mattress, tick.

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