What do we call an organism that lives in or on another organism?

An organism that lives in or on another organism is referred to as a parasite. Parasites depend on their hosts for survival, often benefiting at the expense of the host. This relationship can be detrimental to the host's health, as parasites can consume the host's resources or inflict harm.

In contrast, the other terms describe different types of biological relationships. A predator hunts and kills another organism, known as prey, for survival, while a symbiont refers to organisms that have a mutualistic relationship, benefiting both parties involved. Commensals live on or in another organism but do not harm it, typically gaining benefits such as shelter or nutrients without affecting the host positively or negatively. Thus, the definition of a parasite is specifically tied to the harmful dependence on another organism.