The relationship that exists when one organism benefits at the expense of another is termed parasitism. In this interaction, the parasite derives nourishment or benefits from the host, often harming it in the process. Examples of parasitism include tapeworms in the intestines of mammals or fleas on pets. The parasite typically has evolved adaptations to exploit its host, while the host may develop defense mechanisms to reduce the parasite's impact. This relationship is a key aspect of ecological dynamics and evolutionary biology.