/ kwɔːˈtet; NAmE kwɔːrˈtet/ noun
1.
[ C +sing./pl. v . ] a group of four musicians or singers who play or sing together :
the Amadeus Quartet
2.
[ C ] a piece of music for four musicians or singers :
a Beethoven string quartet
3.
[ C +sing./pl. v . ] a set of four people or things :
the last in a quartet of novels
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WORD ORIGIN
early 17th cent. (in the general sense set of four ): from French quartette , from Italian quartetto , from quarto fourth, from Latin quartus .