What term describes a living organism that obtains food from another organism?

The term that describes a living organism that obtains food from another organism is "parasite." A parasite lives on or within a host organism and derives nutrients at the host's expense. This relationship is typically harmful to the host, as the parasite benefits from the resources of the host without providing any benefit in return.

In contrast to parasites, mutualists engage in a symbiotic relationship that benefits both organisms, commensals usually benefit while the host is neither helped nor harmed, and symbionts can engage in a variety of relationships, including mutualism and parasitism. Therefore, calling a living organism that obtains food from another organism a parasite accurately reflects its nature as a life form that relies on a host for sustenance, often leading to detrimental effects for the host.