Movement of a flat plant part, oriented relative to the plant body and produced by a variety of stimuli that cause disproportioinate growth or increased turgor pressure in the tissues of one surface.

The opening and closing movements of many flowers, and the responses of leaves to changes of temperature and light, are externally directed, or paratonic, nastic movements. Specialized plants, such as the insectivorous sundew, move in response to the touch and chemical stimuli of captured insects.

Nastic movements are responses to stimuli that uniformly affect the plant or else elicit a uniform response regardless of the direction they come from, whereas tropisms are movements in response to stimuli coming from one direction; geotropism, for example, is the response to gravity. The distinction between nasticisms and tropisms is sometimes unclear.

1. The act of controlling a horse quickly with a single strong pull of the reins.
2. The rapid involuntary movement of the eyes that occurs when an image of interest falls on the retina at a distance from the fovea (small depression), as normally occurs when reading the printed page.
3. The series of small, jerky movements of the eyes when changing focus from one point to another.
4. The abrupt rapid small movements of both eyes; such as, when the eyes scan a line of print.

The saccades can be divided into two distinct groups: the major saccades which are easily observed with the naked eye and the minor saccades that are virtually unobservable without special instrumentation.

The word saccade is borrowed from French. It is derived from Old French sachier, "to shake". In horse riding, a saccade is the brusque shaking given to the reins of a horse which is used as a signal to the horse by the rider.