parasite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Biology An organism that lives and feeds on or in an organism of a different species and causes harm to its host.
• noun One who habitually takes advantage of the generosity of others without making any useful return.
• noun One who lives off and flatters the rich; a sycophant.
• noun A professional dinner guest, especially in ancient Greece.

from The Century Dictionary.

• noun Originally, one who frequents the tables of the rich and earns his welcome by flattery; hence a hanger-on; a fawning fiatterer; a sycophant.
• noun Specifically In zoö., an animal that lives in or on and at the expense of another animal called technically the host; also, by extension, an animal which lives on or with, but not at the expense of, its host: in the latter sense, more precisely designated inquilince or commensal (see these words). , Particularly, an insect which lives either upon or within another insect during its earlier stages, eating and usually destroying its host. In botany, a plant which grows upon another plant or upon an animal, and feeds upon its juices. See parasitic, and cut under Cercospora.
• noun In teratology See autosite.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who frequents the tables of the rich, or who lives at another's expense, and earns his welcome by flattery; a hanger-on; a toady; a sycophant.
• noun A plant obtaining nourishment immediately from other plants to which it attaches itself, and whose juices it absorbs; -- sometimes, but erroneously, called epiphyte.
• noun A plant living on or within an animal, and supported at its expense, as many species of fungi of the genus Torrubia.
• noun An animal which lives during the whole or part of its existence on or in the body of some other animal, feeding upon its food, blood, or tissues, as lice, tapeworms, etc.
• noun An animal which steals the food of another, as the parasitic jager.
• noun An animal which habitually uses the nest of another, as the cowbird and the European cuckoo.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology A (generally undesirable) living organism that exists by stealing the resources produced/collected by another living organism.
• noun pejorative A person who relies on other people's efforts and gives little back (originally a sycophant).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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
• noun a follower who hangs around a host (without benefit to the host) in hope of gain or advantage