In general, tropism is an orienting response of an organism to a stimulus. It often involves the growth rather than the movement of an organism. The response exhibited by the organism to the stimulus is mostly involuntary, which means it cannot go either towards or away from the stimulus source as in taxis. In tropism, the response of the organism is often by its growth rather than by its movement. It may grow either towards or away from the stimulus.
Heliotropism is an example of tropism. It is the orienting response to the source of sunlight (or the sun). It may be positive or negative. A positive heliotropism is a response towards the direction of the sun whereas a negative heliotropism is a response away from the direction of the sun. The term heliotropism was first used in 1832 by A. P. de Candolle, a Swiss botanist, to refer to the phenomenon in any plant.
Heliotropism should not be confused though with the other form of tropism, phototropism. The latter is in particular a growth or movement response to the light stimulus.