A lullaby is a slow, quiet song sung to a small child at bedtime, hoping the child will fall asleep. The voice stays soft, the tune often repeats, and the room is usually dark — a repeating gentle tune helps a tired brain let go. Parents, grandparents, and babysitters use it; it feels warm and homely, not formal. Though actually, people also stretch the word a little: a sleepy song playing from a phone can be called a lullaby even when nobody is singing live. As a verb, to lullaby a child is to sing them to sleep that way, but the noun is the use you will meet almost every time. A loud, bouncy children’s song is not a lullaby, even if a child likes it.