noun
secret music; esoteric musical piece
Refers to a piece of music, often in traditional Japanese arts, that is kept secret and transmitted only to select disciples. Not used for everyday 'secret song'.
その流派では、秘曲は一子相伝とされている。
In that school, the secret musical pieces are said to be passed down to only one child.
彼は師から秘曲を授かった。
He was taught a secret piece by his master.