A type of symbiosis where two (or more) organisms from different species live in close proximity to one another, in which one member depends on another for its nutrients, protection, and/or other life functions. The dependent member (the parasite) benefits from the relationship while the other one (the host) is harmed by it.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.