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

Here's a breakdown:

* Parasitism is a type of symbiotic relationship where one organism (the parasite) benefits at the expense of another organism (the host).

* Parasites can be either internal (living inside the host, like tapeworms) or external (living on the host, like ticks).

* The host can be harmed by the parasite in various ways, from mild discomfort to severe illness or even death.

Let me know if you'd like to know more about specific types of parasites!