n. In music: A composition or movement for four solo parts, either vocal or instrumental, usually without accompaniment.n. A company of four singers or players who perform quartets.n. In an orchestra the stringed instruments collectively, and in oratorio music the principal vocal soloists, are sometimes loosely called the quartet.n. A stanza of four lines.n. Same as quadruplet.n. The performers of such a composition, whether vocal or instrumental.n. 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.