piano vs upright piano

piano and upright piano both are nouns.

Word	Noun	Adjective	Verb	Adverb
piano	Yes	Yes	No	Yes
upright piano	Yes	No	No	No

As nouns, upright piano is a hyponym of piano; that is, upright piano is a word with a more specific, narrower meaning than piano:

piano: a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds
upright piano: a piano with a vertical sounding board

Other hyponyms of piano include grand, grand piano, mechanical piano, Pianola, player piano, upright.

piano (noun)	upright piano (noun)
a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds	a piano with a vertical sounding board
(music) low loudness

piano (adjective)	upright piano (adjective)
(used chiefly as a direction or description in music) soft; in a quiet, subdued tone

piano (adverb)	upright piano (adverb)
used as a direction in music; to be played relatively softly