Parasitism
An assortment of definitions
Parasitism is an association, generally continuous, between two different organisms, one of which lives at the expense of the other
M. Caullery - Parasitism and Symbiosis. Sidgwick and Jackson 1952.
Parasitism is a form of symbiosis in which one symbiont, or parasite, receives advantages to the detriment of the other, or host.
Hendersons's dictionary of biological terms (8th edition) Longman Ltd 1975.
Parasitism is a way of living in which an organism, the parasite, uses an organism of a different species, the host, both as a habitat and as a food.
C.P. Reed - Animal Parasitism. Prentice-Hall 1972.
Parasites are those animals which use other living animals as their environment and source of food, at the same time relinquishing to their hosts, partially or completely, the task of regulating their relationships with the external environment.
V.A. Dogiel - General Parasitology. Oliver & Boyd 1964.
Parasitism is an intimate and obligatory relationship between two heterospecific organisms during which the parasite, usually the smaller of the two partners, is metabolically dependent on the host.
T.C. Cheng - General Parasitology. Academic Press 1986.
It is clearly difficult to define parasitism precisely. It is a relationship between to species populations. The essential features are that the parasite is physiologically dependent on the host, that it has a higher reproductive potential than the host, and that it is capable ultimately of killing heavily infected hosts, and that the infections process tends to produce an overdispersed distribution of parasites within the host population.
C.P. Kennedy - Ecological Animal Parasitology. Blackwell Scientific 1975.
A parasite is an organism which has a detrimental effect on the intrinsic growth rate of its host population.
R. M. Anerson & R. M. May J. Animal Ecology 1978.
I resort to Webster's Third International Dictionary for what must be a generally accepted definition: a parasite is an organism living in or on another living organism, obtaining from it part or all of its organic nutriment, commonly exhibiting some degree of adaptive structural modification, and causing some degree of real damage to its host.
P.W. Price - Evolutionary Biology of Parasites. Princeton Univ. Press.