A relationship in which a single celled organisms live within the cells of another organism is called what?
The relationship you are referring to is called endosymbiosis. In endosymbiosis, one organism lives within the cells of another organism, forming a mutually beneficial or parasitic relationship. This association can be observed in various organisms, such as bacteria living inside the cells of plants or animals, or even certain organelles like mitochondria and chloroplasts residing within eukaryotic cells.
To find this answer, you can try searching for the term "relationship in which single-celled organisms live within another organism." This can be done using an internet search engine like Google or Bing, or by consulting science textbooks or reputable scientific websites.