What word describes the movement of a plant, directed toward a light source?

The word you are looking for is phototropism. When a part
of the plant grows toward a light source, this is known as positive phototropism.
Usually, this can be seen as the stem and leaves bend toward the light. Auxins, or
growth hormones, play a role in the elongation and growth toward the light source.
Negative phototropism is seen in the roots, which grow in the opposite direction of the
light, however, gravity acts on the growth of roots in a downward direction.
Phototropism is one of many responses plants can have to external stimuli. Other
tropisms are responses to gravity, water, etc.