What is the term for the part of the ocean that receives sunlight?

The part of the ocean that receives sunlight is called the photic zone.

Also known as the euphotic or sunlight zone, it is the upper layer of seawater where enough light penetrates for photosynthesis. Phytoplankton use that light to produce energy, making the photic zone the foundation of most marine food webs. Fish, zooplankton and many other organisms depend directly or indirectly on this productivity.

The zone’s depth is not fixed. Clear tropical water may transmit light much deeper than muddy coastal water, and seasonal changes in plankton or sediment can alter light penetration. In the open ocean, the photic zone is often described as extending to roughly 200 metres, though conditions vary widely.

The twilight zone lies below it, where some light remains but photosynthesis is limited. The aphotic zone is deeper still and receives little or no sunlight. The photic zone therefore refers to sunlight availability, not simply to the ocean’s shallowest layer.