What is an organism called on which a parasite lives?

An organism on which a parasite lives is called a host. In the relationship between a parasite and its host, the parasite benefits by obtaining nutrients and shelter, while the host may be harmed in the process. This definition is central to understanding the dynamics of parasitic relationships, where the host typically suffers some degree of detriment due to the parasite's presence and activities.

The term "victim" could imply a more general sense of being harmed, but it doesn't specifically denote the biological relationship characterized by parasitism. Similarly, "prey" refers to an organism that is hunted and consumed by another organism (the predator), which is not the case in parasitism. "Commensal" describes a relationship where one organism benefits without affecting the other, which again does not apply to a host-parasite interaction. Each of these terms has distinct meanings in ecological contexts, making "host" the most accurate and appropriate choice.