Undulation

[Sarah]

A motion to and fro, up and down, or from side to side, in anyfluid or elastic medium, propagated continuously among its particles,but with no translation of the particles themselves in the directionof the propagation of the wave; a wave motion; a vibration.

[Sarah]

Undulation in Swimming. The unavoidable but controllable movements of the head, trunk and legs in butterfly. Caused by the overwater arm recovery.

[Jason Yan]

The action of moving smoothly up and down.