The ability to look upon a sheet of music and sing what's written there. The ability has degrees: some people can read enough to follow along; to "fake" the song; to carry it off; and finally, to hear it in one's head.

Some years ago I was in NYC with a pianist friend who is a musical prodigy. He often accompanied singers, and had been asked to learn a particular song. During our pereginations he stopped somewhere and bought the sheet music. Later, at lunch, he pulled the text from his bag, looked it over, and said, "Nice song," as if he had just heard it on the radio.