What term describes the movement of an organism towards or away from a stimulus?

The movement of an organism towards or away from a stimulus is referred to as taxis. This term specifically describes directional movement in response to environmental cues, such as light, chemicals, or gravity. In the case of taxis, the organism may move closer to a positive stimulus (e.g., moving towards a source of light or food) or away from a negative stimulus (e.g., moving away from harmful chemicals).

The other options represent different concepts. Homeostasis refers to the regulation of an organism's internal environment to maintain stable, constant conditions, rather than a directional movement. Phototropism and thermotropism are specific types of growth movements in plants: phototropism is growth in response to light, while thermotropism refers to movement or growth in response to temperature gradients. While these terms are related to responses to stimuli, they do not encompass the broader definition of organismal movement towards or away from stimuli as taxis does.