To undulate is to move up and down in smooth waves, or to have a shape that rises and falls like waves. A snake undulates across the sand. Long grass undulates when the wind crosses a field. A dancer's arms undulate. And land undulates — 'the road undulates through undulating hills' — where nothing is moving at all; the ground just goes gently up, down, up, down like frozen waves. That double life is what makes the word useful: it covers both the motion and the shape the motion would leave behind. The key is 'smooth'. Undulating is not shaking, not bumping, not jumping. It is continuous and rounded, one curve flowing into the next with no corners. If you draw it, you draw a line like the top of a wave, never a zigzag. This is why the word suits water, snakes, silk, wheat fields, and gentle countryside, and does not suit a car on a bad road. The word is somewhat formal and descriptive — you meet it in nature writing, travel writing, geography lessons, and descriptions of dance or music. In everyday Taiwanese-student English you would more often say 'goes up and down' or 'moves like a wave'. The adjective form 'undulating' is far more common than the verb, especially in the phrase 'undulating hills', which is almost a fixed expression in English travel writing.

波動、起伏；（地形）綿延起伏（如「起伏的丘陵」）