biwa song; Song of the Lute
Refers to a narrative song accompanied by the biwa (Japanese lute), or specifically to the famous Chinese poem 'Song of the Pipa' by Bai Juyi, known in Japanese as 琵琶行.
Bai Juyi's 'Song of the Pipa' is one of the masterpieces of Chinese literature.
He played the biwa while reciting a biwa song.
From Chinese 琵琶行 (pípá xíng), literally 'pipa song'. The term was adopted into Japanese with the reading びわこう, referring both to the Chinese poem and to narrative songs accompanied by the biwa.