What do we call an organism that lives in or on another organism and benefits by obtaining nutrients?

The correct term for an organism that lives in or on another organism and benefits by obtaining nutrients from it is a parasite. Parasites derive their nourishment and benefits at the expense of their host, and they can cause harm to the host organism in the process. This relationship is characterized by the parasite relying on the host for survival and growth, often leading to negative effects for the host, such as illness or weakened health.

In contrast, a symbiont refers to any organism that lives in close association with another, which can include both parasitic and non-parasitic relationships. Commensals benefit from the relationship with their host without harming or benefiting the host, while predators actively hunt and kill their prey for food, which is different from the relationship dynamics seen in parasites. Understanding these distinctions helps clarify the unique roles of various organisms within ecosystems.