pianissimo vs piano

pianissimo and piano both are nouns.

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

As nouns, pianissimo and piano are synonyms defined as:

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

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

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