Final answer:
A photosystem is a cluster of pigments embedded in the thylakoid membrane that serves as a light-collecting unit.
Step-by-step explanation:
The cluster of pigments embedded in the thylakoid membrane that serves as a light-collecting unit is known as a photosystem. Within the photosystem, there is a complex of proteins and light-absorbing pigments, such as chlorophyll and carotenoids, that capture light energy for photosynthesis.