What is a Lullaby?

The term “lullaby” comes from medieval songs such as the Coventry Carol . . .

Coventry Carol
Lully, lullay, Thou little tiny Child,
Bye, bye, lully, lullay.
Lullay, thou little tiny Child,
Bye, bye, lully, lullay.
O sisters too, how may we do,
For to preserve this day
This poor youngling for whom we do sing
Bye, bye, lully, lullay.
Herod, the king, in his raging,
Charged he hath this day
His men of might, in his own sight,
All young children to slay.
That woe is me, poor Child for Thee!
And ever mourn and sigh,
For thy parting neither say nor sing,
Bye, bye, lully, lullay.