The Collaborative International Dictionary of English v.0.48:

Piano \Pi*a"no\, a. & adv. [It., even, smooth, soft, fr. L.
   planus even, level.] (Mus.)
   Soft; -- a direction to the performer to execute a certain
   passage softly, and with diminished volume of tone. (Abbrev.
   p.)
   [1913 Webster] Pianoforte

The Collaborative International Dictionary of English v.0.48:

Piano \Pi*an"o\, Pianoforte \Pi*an"o*for`te\, n. [It. piano soft
   (fr. L. planus even, smooth; see Plain, a.) + It. forte
   strong, fr. L. fortis (see Fort).] (Mus.)
   A well-known musical instrument somewhat resembling the
   harpsichord, and consisting of a series of wires of graduated
   length, thickness, and tension, struck by hammers moved by
   keys.
   [1913 Webster]

   Dumb piano. See Digitorium.

   Grand piano. See under Grand.

   Square piano, one with a horizontal frame and an oblong
      case.

   Upright piano, one with an upright frame and vertical
      wires.
      [1913 Webster]

WordNet (r) 3.0 (2006):

piano
    adv 1: used as a direction in music; to be played relatively
           softly [syn: piano, softly] [ant: forte, loudly]
    adj 1: used chiefly as a direction or description in music; "the
           piano passages in the composition" [syn: piano, soft]
           [ant: forte, loud]
    n 1: a keyboard instrument that is played by depressing keys
         that cause hammers to strike tuned strings and produce
         sounds [syn: piano, pianoforte, forte-piano]
    2: (music) low loudness [syn: piano, pianissimo]

The Devil's Dictionary (1881-1906):

PIANO, n.  A parlor utensil for subduing the impenitent visitor.  It
is operated by pressing the keys of the machine and the spirits of the
audience.