Parasitism is a relationship between two species of plants or animals in which one benefits at the expense of the other, sometimes without killing the host organism.
In biology/ecology, parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host. Traditionally parasite (in biological usage) referred primarily to organisms visible to the naked eye, or macroparasites (such as helminths).