symbol vs crossbones

symbol and crossbones both are nouns.

Word	Noun	Adjective	Verb	Adverb
symbol	Yes	No	No	No
crossbones	Yes	No	No	No

As nouns, crossbones is a hyponym of symbol; that is, crossbones 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
crossbones: two crossed bones (or a representation of two crossed bones) used as a symbol danger or death

Other hyponyms of symbol include cornucopia, horn of plenty, death's head, oriflamme, white feather, allegory, emblem.

symbol (noun)	crossbones (noun)
something visible that by association or convention represents something else that is invisible	two crossed bones (or a representation of two crossed bones) used as a symbol danger or death
an arbitrary sign (written or printed) that has acquired a conventional significance