symbol vs death's head

symbol and death's head both are nouns.

Word	Noun	Adjective	Verb	Adverb
symbol	Yes	No	No	No
death's head	Yes	No	No	No

As nouns, death's head is a hyponym of symbol; that is, death's head is a word with a more specific, narrower meaning than symbol:

symbol: something visible that by association or convention represents something else that is invisible
death's head: a human skull (or a representation of a human skull) used as a symbol of death

Other hyponyms of symbol include crossbones, cornucopia, horn of plenty, oriflamme, white feather, allegory, emblem.

symbol (noun)	death's head (noun)
something visible that by association or convention represents something else that is invisible	a human skull (or a representation of a human skull) used as a symbol of death
an arbitrary sign (written or printed) that has acquired a conventional significance