death's head vs symbol

death's head and symbol both are nouns.

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

As nouns, symbol is a hypernym of death's head; that is, symbol is a word with a broader meaning than death's head:

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

Other hypernyms of death's head include symbolic representation, symbolisation, symbolization.

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