n.1. The act or an instance of nodding the head.
2. A wobble in a spinning gyroscope or other rotating body.
3. Astronomy A small periodic irregularity in the precessional motion of the earth's polar axis with respect to the pole of the ecliptic.
4. Botany A slight curving or circular movement in a stem, as of a twining plant, caused by variation in growth rates of different parts.