Source : Webster's Revised Unabridged Dictionary (1913)

Pianissimo \Pi`a*nis"si*mo\, a.[It., superl. of piano.] (Mus.)
   Very soft; -- a direction to execute a passage as softly as
   possible. (Abbrev. pp.)

Source : WordNet®

pianissimo
     adj : chiefly a direction or description in music; very soft

pianissimo
     n : (music) low loudness [syn: {piano}]
     adv : a direction in music; to be played very softly [syn: {very
           softly}] [ant: {fortissimo}]