Parasitism is a type of symbiotic relationship where one species, known as the parasite, benefits at the expense of another species, known as the host. The parasite uses the host for food, shelter, or other resources, often causing harm to the host in the process. Unlike predation, where the predator typically kills its prey, parasitism usually does not result in the immediate death of the host. Instead, the parasite may weaken the host over time, making it more susceptible to disease or other threats.
In summary, the best description of parasitism is: one species uses another species as food or shelter, causing harm.