the cardinal number that is the sum of three and one
"The quartet on her list of favorite numbers sits right after the trio in the sequence."
a set of four similar things considered as a unit
"The quartet of identical lamps lined up along the hallway created a striking visual symmetry."
A music composition in four parts, each performed by a single voice or instrument.
"The choir performed Mozart's famous quartet, where each of the four voices sang an independent melody that wove together beautifully."
In plain English: A quartet is a group of four people who perform together, often as musicians playing instruments or singing songs at once.
"The school choir performed an upbeat quartet that everyone loved."
Usage: A quartet refers to either a musical group of four performers or a piece written specifically for them. Use this term when describing the ensemble size rather than simply saying "four people playing together."
The word entered English as a doublet borrowed directly from the French quartette and its earlier source, the Italian quartetto. It originally referred to a group consisting of four parts or individuals before becoming specifically associated with music.