1. A non-mutual relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host (e.g. Taenia solium). These are now called macroparasites (typically protozoa and helminths). Parasite now also refers to microparasites, which are typically smaller, such as viruses and bacteria, and can be directly transmitted between hosts of the same species. (wikipedia.org)
2. Interaction between two organisms, in which one organism (the parasite) benefits and the other (the host) is harmed. (wiktionary.org)
3. A form of symbiosis in which one organism (called parasite) benefits at the expense of another organism usually of different species (called host). The association may also lead to the injury of the host. (biology-online.org)
4. The relation between two different kinds of organisms in which one receives benefits from the other by causing damage to it (usually not fatal damage). (wordnetweb.princeton.edu)
Word origin: From Latin parasitus “toady, sponger,” and directly from Greek parasitos “one who lives at another’s expense, person who eats at the table of another,” from noun use of an adjective meaning “feeding beside,” from para- “beside” + sitos “food,” of unknown origin.