Plagiarism is the use of ideas, concepts or words from another source
without appropriately acknowledging the source, in a setting where
originality is expected. It is considered academic dishonesty and a
breach of journalistic ethics. To tell if something is plagiarism, use
the guideline: "if you did not write it yourself, you must give
credit".

Plagiarism is not the same as copyright infringement. Copyright
infringement is a violation of the rights of a copyright holder, when
copyrighted material is used without consent.  Plagiarism, in
contrast, is concerned with false claims of authorship. Plagiarism is
considered a moral offence, rather than usually a legal offence.

For example, the works of William Shakespeare and Ludwig van
Beethoven, as well as most early silent movies, are in the public
domain either because they were created before copyright existed, or
because their copyright has expired. It would not be a violation of
copyright to quote Hamlet: "To be, or not to be, that is the question:
Whether 'tis nobler in the mind to suffer the slings and arrows of
outrageous fortune, or to take arms against a sea of troubles", etc., but it
would be plagiarism to pretend that you thought of it yourself.

The word "plagiarism" comes from the Latin word "plagiarius"
(literally "kidnapper"), after the Roman poet Martial complained that
another poet had "kidnapped his verses". The word plagiary was
introduced into English in 1601 by dramatist Ben Jonson to describe
someone who steals others' words.

Software also exists to detect plagiarism. Plagiarism detection
software serves two purposes: to uncover potential plagiarism and to
deter students from plagiarizing. However, plagiarism detection
software doesn't always yield accurate results and there are loopholes
in these systems. For example, simple plagiarism detection programs
can be outwitted by a process called "Rogeting". In its basic form,
Rogeting simply consists of replacing words with their synonyms,
chosen from a thesaurus. A plagiarism checker would not usually be
able to detect the original source; however, the main drawback is that
the new automatically-generated text might not sound natural or might
not make sense at all, thus requiring the intervention of a human
operator.