The Collaborative International Dictionary of English v.0.48:

Quartet \Quar*tet"\, Quartette \Quar*tette"\, n. [It. quartetto,
   dim. of quarto the fourth, a fourth part, fr. L. quartus the
   fourth. See Quart.]
   1. (Mus.)
      (a) A composition in four parts, each performed by a
          single voice or instrument.
      (b) The set of four person who perform a piece of music in
          four parts.
          [1913 Webster]

   2. (Poet.) A stanza of four lines.
      [1913 Webster]

WordNet (r) 3.0 (2006):

quartet
    n 1: the cardinal number that is the sum of three and one [syn:
         four, 4, IV, tetrad, quatern, quaternion,
         quaternary, quaternity, quartet, quadruplet,
         foursome, Little Joe]
    2: four performers or singers who perform together [syn:
       quartet, quartette]
    3: a set of four similar things considered as a unit [syn:
       quartet, quartette, quadruplet, quadruple]
    4: four people considered as a unit; "he joined a barbershop
       quartet"; "the foursome teed off before 9 a.m." [syn:
       quartet, quartette, foursome]
    5: a musical composition for four performers [syn: quartet,
       quartette]