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To cite (citing, citation) means to pinpoint exactly where something you quote came from.
It is generally done twice each time you quote something:
First you include the author's name of the work you are quoting or referencing, and what page what you are quoting/referencing is on in that work.
Second, after your paper/essay is complete, additional pages list each work you quoted in a way that you might find the work and check the author's facts by reading it yourself.
The purpose is to prove to readers that what you are writing is not just your opinion, as you have facts to support what you are writing.
The specific things you need to cite depend on the writing style you are using, which should be chosen for readability and convention of the audience in mind. Check here for more: http://owl.english.purdue.edu/owl/section/2/
verb (used with object), cit·ed, cit·ing.
to quote (a passage, book, author, etc.), especially as an authority: He cited the constitution in his defense.
to mention in support, proof, or confirmation; refer to as an example: He cited many instances of abuse of power.
to summon officially or authoritatively to appear in court.