The term for a relationship in which one species benefits at the expense of another is called parasitism. In a parasitic relationship, the parasite derives nutrients or benefits from the host organism, which may suffer harm or diminished fitness as a result. This relationship can occur in various forms, from external parasites like ticks and fleas to internal parasites like worms. Parasitism plays an important role in regulating host populations and can influence community dynamics within ecosystems. Understanding these relationships is essential for studying ecological interactions and disease transmission.