What is the term for a situation where one organism feeds on or lives off another, typically harming it?

The correct term for a situation where one organism feeds on or lives off another, typically harming it, is parasitism. Parasitism is a specific type of interaction in ecology where the parasite benefits at the expense of the host. This relationship can lead to various negative effects on the host, ranging from a decrease in wellbeing to severe health consequences or even death.

In contrast, commensalism describes a relationship where one organism benefits while the other is neither helped nor harmed. Mutualism involves a relationship where both organisms benefit from the interaction. Symbiosis is a broader term that refers to any close and long-term biological interaction between two different organisms, which can include parasitism, mutualism, and commensalism. Thus, when specifically identifying the harmful interaction described in the question, parasitism is the most accurate term.