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):

pianoforte
    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]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "pianoforte":
   Klavier, Steinway, baby grand, cembalo, clarichord, clavicembalo,
   clavichord, clavicittern, clavicymbal, clavicytherium, clavier,
   concert grand, cottage piano, couched harp, dulcimer harpsichord,
   grand, grand piano, hammer dulcimer, harmonichord, harpsichord,
   manichord, manichordon, melodion, melopiano, monochord,
   pair of virginals, parlor grand, pianette, pianino, piano,
   piano-violin, sostinente pianoforte, spinet, square piano, upright,
   upright piano, violin piano, virginal