An organism that lives inside or on another organism is called a parasite.

Here's a breakdown:

* Parasitism: A relationship where one organism (the parasite) benefits at the expense of another organism (the host).

* Endoparasite: A parasite that lives *inside* the host's body. Examples include tapeworms, roundworms, and some bacteria.

* Ectoparasite: A parasite that lives *on* the surface of the host's body. Examples include fleas, ticks, and lice.

It's important to note that not all organisms that live in or on other organisms are parasites. Some have mutually beneficial relationships (symbiosis). For example, bacteria in your gut help you digest food.