The taking of a share in the resources of one organism by another organism. In a more restricted sense it is an obligatory symbiosis between individuals of two different species, in which the parasite is metabolically dependent on the host and negatively affecting, but rarely killing, the host. As a rule, a more highly organised host and a less organised parasite are involved; thus birds support many 'lower' parasites, but are rarely parasites themselves.

Alternative forms for parasitism : parasitic, parasitises.