/ˈparəˌsīt/
Noun, pl. parasites
(Biology)
1. A (generally undesirable) living organism that exists by stealing the resources produced/collected by another living organism. (wiktionary.org)
2. An organism living in, with, or on another organism in parasitism. (merriam-webster.com)
3.Something that resembles a biological parasite in dependence on something else for existence or support without making a useful or adequate return. (merriam-webster.com)
4. 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. (wordnetweb.princeton.edu)
Other definitions:
(Pejorative)
1. A person who lives on other people’s efforts or expense and gives little back. (wiktionary.org)
2. A person who exploits the hospitality of the rich and earns welcome by flattery. (merriam-webster.com)
(Literary, poetic)
1. A climbing plant which is supported by a wall, trellis etc. (wiktionary.org)
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.