An undulation is one wave-like rise and fall — either a movement or a shape. In the plural, 'the undulations of the sea', 'the undulations of the land', you are describing a whole series of gentle curves going up and down. Used as an uncountable idea — 'the slow undulation of her body as she danced' — it names the wave-like motion itself, the quality of moving in that smooth rolling way. So the word points at two things depending on how you use it: count it, and you mean the individual humps and dips; do not count it, and you mean the rolling motion as a whole. Both keep the same core picture: smooth, rounded, continuous, no corners. Where do you actually meet it? Geography and landscape writing ('gentle undulations in the plain'), descriptions of dance and music, physics and science writing about waves, and careful literary description. It is a formal noun. In ordinary speech people say 'the way it moves up and down' or just 'the bumps' — though 'bumps' loses all the smoothness, which is exactly what the word is protecting. Though actually, you will notice that the adjective 'undulating' does most of this work in real English. 'Undulating hills' is far more common than 'the undulations of the hills', and sounds less heavy. Keep the noun for when you want to point at the curves themselves as things you could count or measure.

起伏、波動；（地形的）起伏波狀