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]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "piano":
   Klavier, Steinway, a cappella, a la sourdine, abbandono,
   accrescendo, adagietto, adagio, affettuoso, agilmente, agitato,
   allegretto, allegro, amabile, amoroso, andante, andantino,
   appassionatamente, appassionato, baby grand, barely audible,
   brillante, capriccioso, cembalo, clarichord, clavicembalo,
   clavichord, clavicittern, clavicymbal, clavicytherium, clavier,
   con affetto, con agilita, con agitazione, con amore, concert grand,
   cottage piano, couched harp, crescendo, decrescendo, dim,
   diminuendo, dimly, distant, dolce, dulcimer harpsichord, faint,
   faint-voiced, faintly, feeble, feebly, forte, fortissimo, gentle,
   gently, grand, grand piano, half-heard, hammer dulcimer,
   harmonichord, harpsichord, hushedly, indistinct, lamentabile,
   larghetto, larghissimo, largo, legato, leggiero, low, manichord,
   manichordon, marcando, melodion, melopiano, monochord, morendo,
   murmured, pair of virginals, parlando, parlor grand, pianette,
   pianino, pianissimo, piano-violin, pianoforte, pizzicato,
   prestissimo, presto, rallentando, ritardando, ritenuto,
   scarcely heard, scherzando, scherzo, scherzoso, soft,
   soft-sounding, soft-voiced, softly, sordamente, sordo,
   sostinente pianoforte, sotto voce, spiccato, spinet, square piano,
   staccato, stretto, subaudible, subdued, subduedly, tremolando,
   tremoloso, trillando, unclear, upright, upright piano,
   violin piano, virginal, weak, weak-voiced, weakly, whispered



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.