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Thanks to Craig Buchek for the content of this page.
Piracy is a term often used to describe the distribution of copyrighted materials, without permission of the owners of the copyrights. It usually refers to large-scale distribution of software, music, or movies, but is also applied sometimes to smaller instances.
Most security experts feel that this is a bad analogy. Real pirates are violent thieves and murderers. People who "pirate" digital media are not doing anything violent, or even depriving anyone of anything, except for hypothetical sales. (And there's good evidence that most of the people getting illegal copies would not have paid for them anyway.) The preferred term to use is "copyright infringement".
Some experts believe that we should think of terrorists as pirates. This is primarily because international law does not have a definition of terrorism as a crime. Piracy laws are still on the books, and in fact piracy on the seas is still a big (US$15 billion a year) problem, especially in the area between the Indian and Pacific oceans.
The Free Software Foundation lists "piracy" as a word to avoid, stating:
Publishers often refer to prohibited copying as "piracy." In this way, they imply that illegal copying is ethically equivalent to attacking ships on the high seas, kidnapping and murdering the people on them.
If you don't believe that illegal copying is just like kidnapping and murder, you might prefer not to use the word "piracy" to describe it. Neutral terms such as "prohibited copying" or "unauthorized copying" are available for use instead. Some of us might even prefer to use a positive term such as "sharing information with your neighbor."