What is plagiarism?

Definition from Dictionary.com: "the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work"
Works Cited entry in MLA format: 

"plagiarism." Dictionary.com Unabridged. Random House, Inc. 14 Oct. 2009. Web.

Definition from the MLA Handbook: "Derived from the Latin word plagiarius ('kidnapper'), to plagiarize means 'to commit literary theft' and 'to present as new and original an idea or product derived from an existing source' (Merriam-Webster's Collegiate Dictionary [11th ed.; 2003; print]). Plagiarism involves two kinds of wrongs. Using another person's ideas, information, or expressions without acknowledging that person's work constitutes intellectual theft. Passing off another person's ideas, information, or expressions as your own to get a better grade or gain some other advantage constitutes fraud. Plagiarism is sometimes a moral and ethical offense rather than a legal one since some instances of plagiarism fall outside the scope of copyright infringement, a legal offense . . ." (52).
Works Cited entry:

MLA Handbook for Writers of Research Papers. 7th ed. New York: Modern Language Association, 2009. Print.

Definition from the APA Publication Manual (2010): "Researchers do not claim the words and ideas of another as their own; they give credit where credit is due (APA Ethics Code Standard 8.11, Plagiarism). Quotation marks should be used to indicate the exact words of another. Each time you paraphrase another author (i.e., summarize a passage or rearrange the order of a sentence and change some of the words), you need to credit the source in the text. . . .

The key element of this principle is that authors do not present the work of a other as if it were their own work. This can extend to ideas as well as written words" (pp. 15-16).

References entry in APA format:

American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: American Psychological Association.