In simple words, in response to stimuli, tropism can be described as the Movement of Plants. The various tropical movements within plants help them adapt and survive to different situations. In response to different stimuli of light,  water , gravity etc they show different kinds of movements and these movements help them to survive in certain conditions.So, when a plant shows a certain growth movement in response to a stimulus, it's called tropism. Tropism will let the plant move in the direction in response to the particular stimulus or reception.