mathematical symbol vs grapheme

mathematical symbol and grapheme both are nouns.

Word	Noun	Adjective	Verb	Adverb
mathematical symbol	Yes	No	No	No
grapheme	Yes	No	No	No

As nouns, grapheme is a hypernym of mathematical symbol; that is, grapheme is a word with a broader meaning than mathematical symbol:

mathematical symbol: a character that is used to indicates a mathematical relation or operation
grapheme: a written symbol that is used to represent speech

Other hypernyms of mathematical symbol include character, graph, graphic symbol.

mathematical symbol (noun)	grapheme (noun)
a character that is used to indicates a mathematical relation or operation	a written symbol that is used to represent speech