Plainsong

Called 'plain' because it is unaccompanied by musical instruments. But those instruments are valued most highly that approximate the human voice (the violin, the electric guitar), for generally speaking musical instruments are tonally and expressively much more limited than a good singing voice. Which is to say: it is instrumental music, without sung vocals, thet ought to be called 'plainsong': singing without instrumental accompaniment should be called 'richsong'.