Noun

(en noun)

Someone who takes advantage of any opportunity to advance his own situation, placing expediency above principle.

* {{quote-news

, year=2011 , date=November 12 , author= , title=International friendly: England 1-0 Spain , work=BBC Sport citation , page= , passage=James Milner's angled free-kick was headed on to the post by the tireless Bent and Lampard the opportunist was perfectly placed to stoop and head in from virtually on the goal-line.}}

Noun

(en noun)

(pejorative) A person who lives on other people's efforts or expense and gives little or nothing back.

(biology) an organism that lives on or in another organism, deriving benefit from living on or in that other organism, while not contributing towards that other organism sufficiently to cover the cost to that other organism.

* {{quote-magazine, date=2013-03

, author= , title=The Smallest Cell , volume=101, issue=2, page=83 , magazine= citation , passage=It is likely that the long evolutionary trajectory of Mycoplasma went from a reductive autotroph to oxidative heterotroph to a cell-wall–defective degenerate parasite . This evolutionary trajectory assumes the simplicity to complexity route of biogenesis, a point of view that is not universally accepted.}}

''Lice, fleas, ticks and mites are widely spread parasites .

(literary, poetic) A climbing plant which is supported by a wall, trellis etc.

* 1813 , (Percy Bysshe Shelley), Queen Mab , I:

Her golden tresses shade / The bosom’s stainless pride, / Curling like tendrils of the parasite / Around a marble column.