The Collaborative International Dictionary of English v.0.48:

Spinet \Spin"et\, n. [OF. espinete, F. ['e]pinette (cf. It.
   spinetta), fr. L. spina a thorn; -- so called because its
   quills resemble thorns. See Spine.] (Mus.)
   A keyed instrument of music resembling a harpsichord, but
   smaller, with one string of brass or steel wire to each note,
   sounded by means of leather or quill plectrums or jacks. It
   was formerly much used.
   [1913 Webster]

   Dumb spinet. (Mus.) See Manichordon.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Spinet \Spi"net\, n. [L. spinetum. See Spinny.]
   A spinny. [Obs.] --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spinet
    n 1: a small and compactly built upright piano
    2: early model harpsichord with only one string per note

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "spinet":
   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, pianoforte, sostinente pianoforte, square piano,
   upright, upright piano, violin piano, virginal