noun.
In embryology, a group of four cleavage-cells considered as a unit, either because they arise at the same time, or because they have a similar origin or prospective function.
noun.
In music: A composition or movement for four solo parts, either vocal or instrumental, usually without accompaniment.
noun.
A company of four singers or players who perform quartets.
noun.
In an orchestra the stringed instruments collectively, and in oratorio music the principal vocal soloists, are sometimes loosely called the quartet.
noun.
A stanza of four lines.
noun.
Same as quadruplet.
noun.
The performers of such a composition, whether vocal or instrumental.