emblem vs device

emblem and device both are nouns.

Word	Noun	Adjective	Verb	Adverb
emblem	Yes	No	No	No
device	Yes	No	No	No

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

emblem: special design or visual object representing a quality, type, group, etc.
device: an emblematic design (especially in heraldry)

Other hyponyms of emblem include colophon, colors, colours, crest, Cross, flag, heraldry, skull and crossbones, totem, totem pole.

emblem (noun)	device (noun)
special design or visual object representing a quality, type, group, etc.	any clever maneuver
a visible symbol representing an abstract idea	an instrumentality invented for a particular purpose
	an emblematic design (especially in heraldry)
	any ornamental pattern or design (as in embroidery)
	something in an artistic work designed to achieve a particular effect