the.com/undulation

means A smooth, wave-like rising and falling motion or shape.

from From Latin 'unda' meaning 'wave' — the same root that gives us 'undulate,' 'inundate' (to flood, to overwhelm with waves), and 'abundance' (originally the sense of waves overflowing). The Latin 'undulatus' pictured something literally wavy, and English borrowed it in the 1600s to describe motion or form that rolls rhythmically, like water, without needing water at all.