<p><strong>Noun</strong></p><ul><li>an act or instance of nodding one's head, especially involuntarily or spasmodically</li><li>Botany: spontaneous movements of plant parts during growth</li><li>Astronomy: the periodic oscillation observed in the precession of the earth's axis and the precession of the equinoxes</li><li>Mechanics: the variation of the inclination of the axis of a gyroscope to the vertical</li></ul>