Photosymbiosis is a type of symbiosis where one of the organisms is capable of photosynthesis.
Examples of photosymbiotic relationships include those in lichens, plankton, ciliates, and many marine organisms including corals, fire corals, giant clams, and jellyfish.
Photosymbiosis is important in the development, maintenance, and evolution of terrestrial and aquatic ecosystems, for example in biological soil crusts, soil formation, supporting highly diverse microbial populations in soil and water, and coral reef growth and maintenance.
When one organism lives within another symbiotically it’s called endosymbiosis.  Photosymbiotic relationships where microalgae and/or cyanobacteria live within a heterotrophic host organism, are believed to have led to eukaryotes acquiring photosynthesis and to the evolution of plants.