Definition:

Parasite: A organism that lives in or on the body of its host without killing the host, but usually debilitating the host to some extent. Dog fleas are examples of parasites; note that they feed as parasites of dogs during their adult phase, while in the larval stages they are not parasitic (and are not on dogs). Lice, on the other hand, are parasites from the time they hatch from eggs throughout their lives (see also parasitoid).