http://www.ingentaconnect.com/content/cabi/pns/1969/00000028/00000001/art00008
I find it ironic how neither of those links work. And the second one is just an article, not an official definition..
Parasitism is a type of
symbiotic relationship between
organisms of different species in which one, the parasite, benefits from a prolonged, close association with the other, the host, which is harmed - Wiki
Main Entry: par·a·site

Pronunciation: \ˈper-ə-ˌsīt, ˈpa-rə-\ Function:
noun Etymology: Middle French, from Latin
parasitus, from Greek
parasitos, from
para- +
sitos grain, food Date: 1539
an organism living in, with, or on another organism in parasitism
parasite
(
păr'ə-sīt')
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n.
- Biology. An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host.
An organism that lives on or
in a different kind of organism (the host) from which it gets some or all of its nourishment. Parasites are generally harmful to their hosts, although the damage they do ranges widely from minor inconvenience to debilitating or fatal disease.
A parasite that lives or feeds on the outer surface of the host's body, such as a louse, tick, or leech, is called an
ectoparasite. Ectoparasites do not usually cause disease themselves although they are frequently a vector of disease, as in the case of ticks, which can transmit the organisms that cause such diseases as Rocky Mountain spotted fever and Lyme disease.
A parasite that lives inside the body of its host is called an
endoparasite. Endoparasites include organisms such as tapeworms, hookworms, and trypanosomes that live within the host's organs or tissues, as well as organisms such as sporozoans that invade the host's cells.
The American Heritage® Science Dictionary