Plagiarism can happen intentionally or otherwise. The writer or author of a certain article may fall in this pit if he/she is not that careful about the phrasing and ideas in his/work. Being thorough would help a lot in avoiding this dreaded term for someone's work: "plagiarized" or for the author to be a "plagiarist". Simply explained/cited here,

Plagiarism, as defined in the 1995 Random House Compact Unabridged Dictionary, as the "use or close imitation of the language and thoughts of another author and the representation of them as one's own original work." Within academia, plagiarism by students, professors, or researchers is considered academic dishonesty or academic fraud and offenders are subject to academic censure, up to and including expulsion. In journalism, plagiarism is considered a breach of journalistic ethics, and reporters caught plagiarizing typically face disciplinary measures ranging from suspension to termination.

Authors and writers, no matter how professional they are, may misuse sources at certain times in their profession by failing to properly acknowledge these sources. When writing a paper, if a writer/author presents a material, whether directly or otherwise, taken from the work of someone else and do not acknowledge such use of material or idea, plagiarism is already in the air: it is in the work!

NOT plagiarizing someone else's work means NOT using either the words or the ideas of another writer without clearly showing whose original work it is/they are and stating or citing exactly where they come from.

The word plagiarism derives from the Latin word, which means “kidnapping”. In the olden times plagiarii were pirates that sometimes "kidnapped" or stole children. As stated earlier, in the introductory paragraph, plagiarism in academic media signifies the purposeful or accidental unaccredited use of the source material by other writers.

Take note that intentional plagiarism is simply dishonest, and it is theft, thus unacceptable and unethical. It is a form of cheating, and it will be heavily penalized. Unintentional plagiarism which is caused by carelessness and irresponsible review of work at hand or occurs due to the lack of understanding of the topic of work, is equally unacceptable and unethical.