What does the word quartet mean?

Usage examples for quartet

  1. The quartet sauntered out into the ballroom just as the signal for the grand march was given. – The Chase of the Golden Plate by Jacques Futrelle
  2. A quartet of capital voices were singing, and one who joined the chorus, standing by the flag, absently yet caressingly spread it at such breadth that we easily read on it the name of the command. – The Cavalier by George Washington Cable