A creature that lives off another organism is a parasite. The parasite might not hurt the host, but it doesn't do anything to help it, either.

If you call someone a parasite, you are really hurling an insult! The Ancient Greeks used the word parasitos to describe someone who ate at your table but never invited you back. Some telling synonyms include leech," toady, sponge, and hanger-on." Nice, huh? Some crafty birds, such as the cowbird or cuckoo, are called social parasites, laying their eggs in other birds' nests and expecting those mamas to raise their young for them. Unfortunately, there are some people like that, too.

Definitions of parasite
  1. noun
    an animal or plant that lives in or on a host (another animal or plant); it obtains nourishment from the host without benefiting or killing the host
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    an animal or plant that nourishes and supports a parasite; it does not benefit and is often harmed by the association
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    endoparasite, endozoan, entoparasite, entozoan, entozoon
    any of various parasites that live in the internal organs of animals (especially intestinal worms)
    ectoparasite, ectozoan, ectozoon, epizoan, epizoon
    any external parasitic organism (as fleas)
    parasitic plant
    plant living on another plant and obtaining organic nutriment from it
    any wingless bloodsucking parasitic insect noted for ability to leap
    Parasitaxus ustus, parasite yew
    rare and endangered monoecious parasitic conifer of New Caledonia; parasitic on Falcatifolium taxoides
    Buckleya distichophylla, buckleya
    parasitic shrub of the eastern United States having opposite leaves and insignificant greenish flowers followed by oily dull green olivelike fruits
    Comandra pallida, bastard toadflax
    woody creeping parasite of western North America having numerous thick powdery leaves and panicles of small dull-white flowers
    Pyrularia pubera, buffalo nut, rabbitwood
    shrub of southeastern United States parasitic on roots of hemlocks having sparse spikes of greenish flowers and pulpy drupes
    Loranthaceae, family Loranthaceae, mistletoe family
    in some classification includes Viscaceae: parasitic or hemiparasitic shrublets or shrubs or small trees of tropical and temperate regions; attach to hosts by haustoria
    Loranthus europaeus, mistletoe
    shrub of central and southeastern Europe; partially parasitic on beeches, chestnuts and oaks
    American mistletoe, Arceuthobium pusillum
    small herb with scalelike leaves on reddish-brown stems and berrylike fruits; parasitic on spruce and larch trees
    Christmas tree, Nuytsia floribunda, fire tree, flame tree
    a terrestrial evergreen shrub or small tree of western Australia having brilliant yellow-orange flowers; parasitic on roots of grasses
    Old World mistletoe, Viscum album, mistletoe
    Old World parasitic shrub having branching greenish stems with leathery leaves and waxy white glutinous berries; the traditional mistletoe of Christmas
    false mistletoe, mistletoe
    American plants closely resembling Old World mistletoe
    hemiparasite, semiparasite
    a parasitic plant that contains some chlorophyll and therefore is capable of photosynthesis
    type of:
    being, organism
    a living thing that has (or can develop) the ability to act or function independently
  2. noun
    a follower who hangs around a host (without benefit to the host) in hope of gain or advantage
    synonyms: leech, sponge, sponger
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    type of:
    a person who accepts the leadership of another
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