What is the term for the movement of a stem towards sunlight and the movement of the root away from it?

Phototropism is the growth or movement of an organism in response to light. 

The movement exhibited by a stem that grows toward sunlight is called positive phototropic movement. This allows for maximal light absorption to aid in photosynthesis.

On the other hand, the roots grow in the opposite direction of the light source, which is known as a negative phototropic movement. This allows for roots to anchor the plant into the soil and absorb water and nutrients from the soil.