nutational

noun

1. the act or instance of nodding the head
2. a periodic variation in the inclination from the vertical of the rotation axis of a spinning body, as a top
3. Astron. such a small, periodic oscillation of the earth's axis which slightly varies or nods the earth's precessional motion
4. Bot. a slight rotatory movement, as in the stem of a plant, due to the varying rates of growth in its parts