From 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]