What is an organism that lives on or in another organism and derives its nutrition from that organism?

A parasite

Explanation:

Parasitism is a form of symbiosis where one organism lives on/in another organism, as in the question, and gets a benefit from that organism, while the other organism is harmed. This is denoted by + / -, which means that one is positively impacted and one is negatively impacted, as opposed to commensalism, which is + / 0, or mutualism, which is + / +.

An example is tapeworms, which are flatworm parasites that live in the human intestine; they benefit while the host suffers.