Nastic movement is a non-directional movement of a plant part in response to an external stimulus, such as temperature, humidity, light irradiance, touch, or contact. Unlike tropic movements, which are directional and growth-dependent, nastic movements are non-directional and may or may not be growth-dependent. The movement can be due to a change in the turgor pressure, where a decrease in turgor pressure results in shrinkage, while an increase in turgor pressure causes swelling.

Plants exhibit five forms of nastic movements:

• Seismonastic movement: response to mechanical stimuli, such as touch or vibration.
• Photonastic movement: response to light.
• Thermonastic movement: response to temperature.
• Nyctinastic movement: response to the daily light-dark cycle.
• Thigmonastic movement: response to touch or contact.

Examples of nastic movements include the folding of leaves in the touch-me-not plant (Mimosa pudica) when touched, the opening and closing of flowers in response to light or temperature changes, and the movement of the Venus flytraps leaves in response to touch.