Sine wave
The smoothest wave there is: the shape of a swing, a ripple or a pure tone.
See it in Mesh warp. Pick Wave under Mode and watch the grid ripple.
In plain words
Picture a point moving round a circle at a steady speed. Its height goes up, down and up again in a smooth curve. That curve is a sine wave. A swing, a bouncing spring and a pure musical note all follow it. Animation uses sine waves for anything that should sway or pulse without sudden changes.