Which interaction describes one species benefiting at the expense of another, which is harmed but not killed?

The interaction that describes one species benefiting at the expense of another, which is harmed but not killed, is parasitism. In this relationship, the parasite derives nourishment or some other benefit from its host while the host is negatively affected, often suffering from a decline in health, resources, or reproductive success. A common example is a tick feeding on the blood of a mammal; the tick benefits by obtaining nourishment, while the host may experience irritation, disease transmission, or even more severe health issues.

In contrast, other ecological interactions don't fit this description. For instance, commensalism involves one species benefiting while the other is neither helped nor harmed, and predation refers to a predator killing and consuming its prey, resulting in the death of the latter. The neutral interaction indicates that neither species benefits nor is harmed, which does not reflect the imbalance present in parasitism. Thus, the definition of parasitism aligns perfectly with the characteristics given in the question.