fōˈtä.trəˌpizəm noun
Etymology: International Scientific Vocabulary phot- + -tropism
1. : a tropism in which light is the orienting stimulus (as in the turning toward a light of a plant shoot or a tube worm and in the creeping away from a light of a blowfly larva) — compare heliotropism
2. : the reversible change in color of a substance produced by the formation of an isomeric modification when exposed to radiant energy (as light)