Much of what follows is derived from this
Wikipedia article, and I highly recommend reading the entire Wikipedia entry on this topic as it makes for fascinating reading and historical perspective.
With the exception of an accusation of outright fraud, ie. falsified data, I cannot think of anything that would stimulate an instantaneous fear reflex in an author more rapidly than an accusation of plagiarism.
The definition of plagiarism is not straight forward but according to Wikipedia is approximately this:
Plagiarism is defined as the "wrongful appropriation," "close imitation," or "purloining and publication" of another author's "language, thoughts, ideas, or expressions," and the representation of them as one's own original work. In fact, the word plagiarim derives from the Latin word plagiarius, which means "kidnapper."
Interestingly, plagiarism is a relatively new concept in the history of literature and prior to the 18th century literature was considered, at least by many to be common property that could be borrowed or copied without attribution. Even today plagiarism is tolerated and even encouraged in certain artistic endeavors.
Although plagiarism has moral and ethical consequences, there are no legal implications. According to Wikipedia, plagiarism is not currently mentioned in ANY statute or law, either criminal or civil. It should not be confused with copyright infringement, which DOES have legal significance.
However, in the worlds of scientific research and journalism, plagiarism carries significant moral and ethical consequences. Severe sanctions are likely to be levied against those found guilty of such infractions.
In today's internet and computer intense world, plagiarism is both easier to perform and easier to detect. Software programs, loosely referred to as plagiarism detectors, are widely available. A decade ago, a physics professor here at the University of Virginia, Lou Bloomfield, created a "plagiarism detector," ran his students' semester essays from his introductory physics course through it and detected a shockingly high rate of plagiarism. The resultant stir made the press
nationally and locally resulted in honor trials for 122 students.
A decade later, professor Bloomfield maintains a
web site devoted to plagiarism and distributes free software for its detection.
Editorial Manager, the software package used for manuscript submission and review by AJSP has recently added a "plagiarism detector" known as CrossCheck, which the editor may access to search for blocks of text copied without attribution from prior publications. As might be expected, processing a manuscript through this software does take some time.
Some ethicists have argued that in today's internet "copy and paste" world students are not properly educated regarding what constitutes plagiarism. Like pornography, it can be difficult to define. It has been suggested that more attention be paid to teaching the concept of plagiarism in secondary school education.
The concept of "self plagiarism" is an interesting one. If you re-read the definition of plagiarism above, it may be argued that self plagiarism is an oxymoron. Nonetheless the term has recognized meaning and basically refers to "duplicate publication." No theft from others is involved, but there is an element of intellectual dishonesty.
The best protection against plagiarism is the liberal and appropriate use of reference footnotes, even if those footnotes are to your own prior writings.