Symbiosis (pl. symbioses) means living together. It describes close and long-term relationships between different species. The term wasused by Anton de Bary in 1879, as "the living together of unlike organisms".
A symbiont is an organism living in a relationship with another species in which one or both get benefits. When one species lives inside another species, or a microscopic symbiont lives inside the cells of a host, it is called an endosymbiont.