grand piano vs pianoforte

grand piano and pianoforte both are nouns.

Word	Noun	Adjective	Verb	Adverb
grand piano	Yes	No	No	No
pianoforte	Yes	No	No	No

As nouns, pianoforte is a hypernym of grand piano; that is, pianoforte is a word with a broader meaning than grand piano:

grand piano: a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs
pianoforte: a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds

Other hypernyms of grand piano include forte-piano, piano.

grand piano (noun)	pianoforte (noun)
a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs	a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds